In whay country was this purchased in? Some countries, such as Saudi Arabia, UAE, ban FaceTime. If the phone was purchased in such a country, it cannot be added.
Otherwise, here is another suggestion: First, go to Settings, then General, then Restrictions, and verify that FaceTime was not turned off.
Hopefully, you are also aware that the iPhone does not have a dedicated FaceTime application, but rather, it is accessed through the Contacts app after selecting a contact.