Hello there I am getting 'the type or namespace 'Controller' could not be found (are you missing an assembly reference?'

I am using Unity 2019.3 and also cannot find the script when I assign it to a character. I saw using System.Web.Mvc can fix it but it cannot find the package to install it. Any help what should I do? and What to learn for future similar issues? Like learning about libraries, packages... Thanks