What was the result when you followed the guidance in the Related Article above your name on this page, that you were reading just before you posted here? If the Related Article doesn't answer your question please post back with more details about the issue and another user in this user-to-user forum will try to help.
Specifically, under What To Do Next:
If you still can't enroll your face or unlock your device with Face ID, take your device to an Apple Store or Apple Authorized Service Provider, or contact Apple Support about your iPhone or your iPad.