string in namespace std does not name a type

Question

You need to

#include <string>

<iostream> declares cout, cin, not string.

Nouns.h doesn't include <string>, but it needs to. You need to add

#include <string>

at the top of that file, otherwise the compiler doesn't know what std::string is when it is encountered for the first time.

You need to add:

#include <string>

In your header file.