I am working on some old code using the C++ Builder v6 IDE. I added a *.cpp file to a package. Normally when you click on the cpp file the code opens with tabs at the bottom that switch between the .cpp and the .h file of the same name.
I’ve never used C++ Builder, but from your account, it doesn’t seem there’s anything to fix.
If you added a single .cpp file, perhaps there’s no corresponding .h one.
Does that .cpp file #include and .h file with the same name?