iPhone back tap doesnt work

As it is a physical gesture, it is difficult to guide you concretely.

Tap 2 or 3 times, not too quickly but not too slowly either. Yes, I know hehe.

Also try tapping a little under the camera.

Which functions have you chosen to be used as Back Tap functions?

Back Tap can be a bit inconsistent. You need to tap slightly above the leaf on the Apple Logo on the back of your iPhone.

Remember if the function requires the phone to be unlocked, Back Tap will not work on a locked screen.