Ok, so not related to Npp at all I guess.
The error message is quite clear in your case,
g++: error: Num.cpp: No such file or directoryg++ tries to find the file Num.cpp but failed because
it doesn’t exists, it isn’t in the same directory as your main.cpp,
isn’t in a directory where PATH variable points to and
isn’t in a directory where your include statements point to.
But in general I would think that if you post c/c++ programming questions
to a dedicated c/c++ programming forum, you might get faster and better answers.
This forum is mainly about using/working with notepad++
Cheers
Claudia