How to install runtime extensions for DotNet 5

Question

I just installed DotNet 5.0, but I realized that I could not use any of the function from the runtime extensions. The program crashes every time I call any function from Random, Environement or Math, with the error System.TypeLoadException: Could not resolve type with token 01000064 from typeref (expected class 'System.Environment' in assembly 'System.Runtime.Version=5.0.0.0' ...).

I tried to install this by typing the command dotnet add package System.Runtime.Extensions --version 4.3.1 but the program still crashes.

Any help would be greatly appreciated.