Unit test in C# not able to add Reference of other solution which is .net6

whats the solution of error .
targets ‘net6.0’. It cannot be referenced by a project that targets ‘.NETFramework,Version=v4.8.1’

Please provide more info like screen shots of exact and complete message.
Overall .NET Framework and .NET Core (which .NET 6 belongs to) are incompatible.
You can add references of the same framework or .NET Standard in both.

I did a quick demo code.

.NET Framework app

It is a .NET Framework console app.

I wrote 3 class libraries respectively with .NET Framework, .NET Standard and .NET (Core).

The 2 First will work perfectly.

The 3rd won’t even be allowed by IntelliSense (In VS).

namespace DotNetFConsole
	internal class Program
		static void Main(string[] args)

But if you force it and try to compile

.NET 6 app

Works on that direction

I must admit I am a bit surprised because .NET Framework code should not work AFAIK. Maybe this code is too simple to cause any issue.
Or stated another way, there is a compatibility layer that can handle such simple code.

Hope this helps at least understand your problem.