To test this I created a new UWP app project.
The Target Version and Minimum Version of the project are both "Creators Update Build 15063".
I simply added a new .NET Core class library project to the solution and attempted to reference the .NET Core class library from the UWP project and the error appeared.
Also note that I even updated the Microsoft.NETCore.UniversalWindowsPlatform nuget package version in the UWP project to version to v5.3.3. The same error appears.
What gives? O_o