Beware the global.json file

 If you are trying to get a .NET Core project to build, beware that if you have a global.json file in your user folder, it might be overriding any other settings. 

Warning signs are yellow exclamation mark icons in your References and Dependencies, including NuGet (because if you don't have the right SDK you can't get far enough to start updating references).

"The current .NET SDK does not support targeting .NET Core 2.1. Either target .NET Core 1.1 or lower, or use a version of the .NET SDK that supports .NET Core 2.1".

probably could also be any combination.....

"The current .NET SDK does not support targeting .NET Core 2.2. Either target .NET Core 2.1 or lower, or use a version of the .NET SDK that supports .NET Core 2.2".

"The current .NET SDK does not support targeting .NET Core 2.3. Either target .NET Core 2.2 or lower, or use a version of the .NET SDK that supports .NET Core 2.3".

File:

MicrosoftNET.TargetFramework

Line 112