Visual Studio 15.8.1 not running MS unit tests

Question

When I updated Visual Studio to the latest version, 1 of my test projects stopped running tests and outputted this message:

Test project {} does not reference any .NET N

Answer 1

I know that it is stupid, but for me nothing from earlier answers were working. After that I just restart my computer and everything is working correctly :) (My proble was that one day everything was working correctly and next day it stopped working)