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Hi @30602091 ,
Welcome to Microsoft Q&A!
The error message shows that the environment of your .NET 6.0 project is .net6.0 and the environment of the UWP project doesn't support .net6.0. So the UWP project is not compatible with the .NET project.
If you want to use Windows Runtime APIs, you could just make some confirugation in for your .NET 6.0 porject so that you could directly call Windows Runtime APIs in your app. You will need to Edit Your Project File and Modify TargetFramework Element to net6.0-windows10.0.19041.0.
You could check this document - Call Windows Runtime APIs in desktop apps for detailed steps.
Thank you,
Junjie