It’s not a matter of the green line.
You can add in the missing function in the cpp but it’s not going to be called anyway (only the implementation variant will be called by the engine)
My own projects are running fine without the cpp implementation of the base function.
You need to run the project from your IDE not from the project icon once you go c++.
The .gen file is the file generated for unreal. Do your modifications in the header file .h or c++ file .cpp