Why can't I share my screen or see others screens during FaceTime on iPhone?

Whenever I’m on FaceTime with anybody it never lets me share my screen. It also never lets me see other peoples screen, I’ve tried everything and it just doesn’t work! Can you help me?!?!

[Re-Titled by Moderator]

Original Title: my phone wont let me screen share or let me see other peoples screeens when they screen share