You’ve not included <string> header file in stock.h header file, even though you’re using std::string in it. Maybe that is causing this error message (if that is the case, then I would say its really a bad message).
Another problem is that in Stock class definition, you’ve written this:
bool Stock::operator > (Stock & aStock);
which is wrong. Remove Stock:: from it, and make it like this:
bool operator > (const Stock & aStock);
//^^^^ add this also (better)
Stock:: is required when defining the function outside the class.