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Can't create package for WinUI 3 Packaged app with plain .NET library

I have WinUI 3 Packaged app targeting .NET 6 Windows with some libraries targeting .NET 6. With Debug everything is okay, but when I want create package (Solution Explorer -> Package project -> Publish -> Create app packages...) I get error, when I can't create package, because libraries don't have target for specific .NET from package (.NET6.0-windows...). Currently I solved it with duplicated libraries targeting windows .NET, but for future development it is absolutely impractical.
Error message:
Assets file '{Path to specific project}\obj\project.assets.json' doesn't have a target for 'net6.0-windows10.0.19041.0'. Ensure that restore has run and that you have included 'net6.0-windows10.0.19041.0' in the TargetFrameworks for your project.
Is any solution for this problem?

Simply multi-target your library-projects like changing
<TargetFramework>net6.0</TargetFramework>
to
<TargetFrameworks>net6.0;net6.0-windows10.0.19041.0</TargetFrameworks>

Which PCL profile should I use in my Xamarin.Forms app?

I'm new in Xamarin, Xamarin.Forms and C# platform.
I'm starting the development of an app that should support Android and iOS devices. I'm using a PCL project for code sharing. Currently this project is using the profile111 because Visual Studio has selected it by default for me. But when I try to install some NuGet packages I'm getting a message saying the package is not compatible. Something like:
Could not install package 'DocsBrStandard 2.0.1'. You are trying to
install this package into a project that targets
'.NETPortable,Version=v4.5,Profile=Profile111', but the package does
not contain any assembly references or content files that are
compatible with that framework.
I believe this message is related with the profile I'm using, right?
So which profile should I use for my app that should support only iOS and Android devices? Is profile111 the more appropriate for this scenario?
At least, how can I identify if a package is compatible or not with my Xamarin.Forms app?
Thanks.

You need to target .NET Standard to be able to use the DocsBrStandard package. So you can't work with profile based PCL if you want to use that package.
Check out this article about targeting .NET Standard with Xamarin.Forms:
https://blog.xamarin.com/building-xamarin-forms-apps-net-standard/

Added reference not detected in visual studio

When trying to install the netmq vis nuget, I get the following error:
Could not install package 'AsyncIO 0.1.18'. You are trying to install this package
into a project that targets '.NETPortable,Version=v4.5,Profile=Profile111',
but the package does not contain any assembly references or content files that
are compatible with that framework. For more information, contact the package author.
The problem is that netmq depends on AsyncIO. when installing it, nuget find that the assembly is not compatible with .Net 4.5.
so nuget fail in installing AsyncIO, and then failed in installing netmq.
So I downloaded the AsyncIO Source from Github and build it locally with .Net 4.5.
After that and added the dll of AsyncIO built locally as a reference for my project.
Theoretically, NetMQ should be successfully installed with nuget. because I added the needed reference to AsyncIO.
But when trying to reinstall NetMQ, I get the same error:
Could not install package 'AsyncIO 0.1.18'. You are trying to install this package
into a project that targets '.NETPortable,Version=v4.5,Profile=Profile111',
but the package does not contain any assembly references or content files that
are compatible with that framework. For more information, contact the package author.
and nuget did not detect that I added 'AsyncIO 0.1.18' in my project.
How to let nuget detect that I added this reference in my project?

Have a look here:
.NET Portable profiles
Profile111 is a combination of:
.NET Framework 4.5
Windows 8.0
Windows Phone 8.1
or in other words:
portable-net45+netcore45+wpa81
So your project into which you are trying to append the NuGet targets Windows Phone 8.1 and other 2 platforms, I mention this one as the most restrictive.
Now let's have a look at the named NuGet package's source:
<ProjectGuid>{3830B7A3-0225-4FDA-B155-E085E183650C}</ProjectGuid>
<OutputType>Library</OutputType>
<TargetFrameworkVersion>v4.0</TargetFrameworkVersion>
<TargetFrameworkProfile>
</TargetFrameworkProfile>
What do we have here? We can see that the project is not a PCL. It is targeting full .NET 4.0 framework which is not available on Windows Phone 8.1. And your library does target Windows Phone 8.1. See the problem?
You say that if you compile the AsyncIO targeting .NET 4.5 than you can successfully append it into your project as a reference? That's not altogether true. I mean you can append a reference to it but you cannot use it. You will see that when you try to call something from that reference.
In order to use AsyncIO from you PCL will have to build AsyncIO as a PCL targeting the same or more restrictive set of platforms. Try creating a PCL project targeting Profile111 and try compiling the AsyncIO code with it (just link the original AsyncIO source files (*.cs) into this new AsyncIO_PCL project). If you are lucky enough and the code of AsyncIO is really compatible you will be able to use that library.
Here are your steps:
Create a new project (named AsyncIO_PCL) of type PCL class library.
Pick the Profile111 platform set i.e.
.NET Framework 4.5
Windows 8.0
Windows Phone 8.1
Link all the .cs files from the original AsyncIO project save one (AssemblyInfo.cs) into the new AsyncIO_PCL project.
Set the output assembly name to the same as in original AsyncIO project.
Try building the project.
Your ability to build the AsyncIO as a PCL with the required set of supported platforms depends on AsyncIO code i.e. which API is used inside and if that API is supported by all the three platforms you are targeting.

Try clicking right-button on the project -> Properties. In the "Application" menu check the "Target Framework". It should be set to .NET Framework 4.5

Cannot add the Signalr Client library as a portable library to a Xamarin project

I have tried adding the Signalr Client library and the WebApi client package to my Xamarin Solution (on Visual Studio with Xamarin Support and also on Xamarin Studio) but I always get this error although I am able to install other PCLs (for example Microsoft.Net.Http):
Could not install package 'Microsoft.AspNet.WebApi.Client 5.2.3'. You are trying to install this package into a project that targets 'portable-net45+sl50+win+wpa81+wp80+MonoAndroid10+xamarinios10+MonoTouch10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
and the following error when adding Signalr:
Adding Microsoft.AspNet.SignalR.Client...
Adding 'Microsoft.AspNet.SignalR.Client 2.2.0' to Corda.Client.
Could not install package 'Microsoft.AspNet.SignalR.Client 2.2.0'. You are trying to install this package into a project that targets 'portable-net45+sl50+win+wpa81+wp80+MonoAndroid10+xamarinios10+MonoTouch10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.
I have also tried changing the the target framework with not success.
This is the build setting for my project:
and this is my C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETPortable\v4.5\Profile\Profile259\SupportedFrameworks folder:
I get the same errors on both Mac OSX and Windows 10. Any ideas?

Using Profile78 solved my problem. Full explaination here.

A simple workaround that worked with my Xamarin with VS:
Download the dll-files manually here: https://components.xamarin.com/gettingstarted/signalr/true
Import the following into your solution.
Microsoft.AspNet.SignalR.Client.dll
Newtonsoft.Json.dll
System.Net.Http.Extensions.dll
System.Net.Http.Primitives.dll
Done!
The problem is not with the dll-files, but with the (nuget) installer.

The problem is with the nuget package manager,specifically with the Windows Phone 8.1 project. I think it expects it to be a windows phone silverlight 8.1 project. There is a bug on installing signalr client on an universal 8.1 phone project in a xamarin project. It should be sorted out in the next xamarin release. Unselecting windows phone 8.1 in the portable config solves the issue,meaning you will have to take the windows phone 8.1 project out of the xamarin project and your need to create a universal windows 10 project.

Yes, As suggested by others this is the problem from nuget package manager, As per this link it should be fixed with the realease of Microsoft.signalr.client 2.2.1 version but for now if you still want to go with nuget, Add the below nuget source
https://www.myget.org/F/aspnetwebstacknightly/
to visual studio and install pre release version 2.2.1 pre-160523-b816 of the signalr client dll as suggested in below link
https://github.com/SignalR/SignalR/issues/3393

How to make a NuGet package for this PCL library

I'm trying to create a nuget package for a PCL library I've made.
This nuget package is intended to be used in a Xamarin Forms application though. So I'm not even sure if the things I've ticked are correct.
I really only care about .NET Framework 4. That is my bare minimum framework. For the rest, I don't care at all and have no idea what I should be targetting.
For what it's worth, I'm using NuGet Package Explorer to create my NuGet nupkg files to manually test them before I publish them to NuGet.
Here's a screenshot of what I was trying, but Xamarin Studio failed to add the nuget package (saying it wasn't the right something-or-rather).
and the error message from Xam Studio:
Adding Foo...
Adding 'Foo 1.3.0' to Core.
Could not install package 'Foo 1.3.0'. You are trying to install this package into a project that targets 'portable-net45+win+wp80+MonoTouch10+MonoAndroid10+xamarinmac20+xamarinios10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author.

For Xamarin.Forms the two profiles that are good ones for your Portable Class Library (PCL) project to use are Profile 78 or Profile 259. Xamarin.Forms can be used in other profiles but if your PCL project targets one of these you should be OK. Your project looks to be targeting Profile 78.
Here are the Xamarin.Forms PCL profiles for reference.
Xamarin.Forms 1.3:
portable-win+net45+wp80+MonoAndroid10+MonoTouch10
Xamarin.Forms 1.4:
portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10
The problem with your NuGet package that it does not indicate that it supports Windows 8 in the PCL. NuGet will look at all the individual frameworks that make up the project's PCL profile (ignoring the Xamarin and Mono ones since they are optional if you do not have them in your NuGet package's PCL profile) and ensure that the NuGet package's PCL profile has a framework which is compatible. If the NuGet PCL does not have a matching framework then NuGet considers it incompatible.
So your NuGet package's portable library folder would need to include win
portable-net4+sl5+wp8+win
Then you can install your NuGet package into your Profile 78 PCL project.