Well, in my case I was not trying to debug the .Net framework, but I was getting the same error: Cannot find .cs files for debugging .NET source code. So
I had to turn on the “Enable just my code” option under:
Tools -> Options -> Debugging -> General -> Enable just my Code
Per MS docs:
You can configure Visual Studio to automatically step over system, framework, and other non-user calls and collapse those calls in the call stack window.
https://docs.microsoft.com/en-us/visualstudio/debugger/just-my-code