You're digging large holes for yourself.
> #include <stdlib.h>
Use #include <cstdlib>
> #include "MenuTransition.cpp"
No, include the .h file, not the source file.
If you include the source file, you're soon going to run into multiply declared errors.
> using namespace std;
No, do not import the namespace in a header file.
If you really need to do this, then put it in each .cpp file.
> void combatMenu()
Put the implementation in BattleMenu.cpp
Do you have any idea how "MenuTransition" and "BattleMenu" relate to one another?
A few hours of sketching out your framework of how all these things relate to one another will save you weeks of random hacking in the hope that something will eventually work.
Once you get past the student homework "single file, single edit" stage and onto needing multiple source files, you need a plan of some sort.
Hack, hope, and dump on a forum when it doesn't work is not a long term strategy.