cout << "this does not add anything and returns a constant" << endl;
are two distinct functions. If you meant sum(1,3) and you called sum() you get the wrong answer.
The compiler things you are trying to do the above, but you don't have both versions of the function so it becomes confused (and you did not want 2 versions of the function, you just don't understand your syntax error).
you need to call countdown(10,2); //or whatever variables or numbers you need in there.