I bought my phone at T-Mobile and now I’m receiving a message stating my camera is fake , why ?

If you have not had repair work done on this phone, I suggest that you take it to the nearest Apple Store and have them explain and clear up this message.

Apple Store - Find a Store - Apple

If you haven't replaced the camera and are getting that message, then it means there is a hardware issue with the camera.

Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple