Error CS0012: The type 'Object' is defined in an assembly that is not referenced

You're trying to compile your project and get the following error:

The type 'Object' is defined in an assembly that is not referenced. You must add a reference to assembly 'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51

Adding a reference to NETStandard.Library doesn't help. Probably, that means that you're using .NET Framework and .NET Standard in one solution. Your solution is to avoid using them at the same time since Microsoft doesn't support it. For more information, please, refer to this GitHub issue.