You want public inheritance:
class Weapon : Shopable
should be:
class Weapon : public Shopable
Also, names like _SHOPABLE_H_ are illegal in user written C++ code, as they are reserved for the C++ implementation. Forget the leading underscores and use SHOPABLE_H.
And:
Weapon(int Cost,int Damage,std::string Name)
should be:
Weapon(int Cost,int Damage, const std::string & Name )
to avoid the unnecessary overhead of copying the string.
You might want to rethink your naming convention – typically, function parameter names in C++ begin with a lower case latter. Names beginning with uppercase letters are typically reserved for user-defined types (i.e. classes, struct, enums etc.)
As a matter of interest, which C++ textbook are you learning from?