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 was trying to run an existing project. I had the .NET Core SDK 3.3 installed, but did not have 2.1 installed. This was causing the tests to silently fail.