How do I able Face ID backb

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.

This is usually a hardware failure, so if the troubleshooting steps in the Related Article don’t fix it use the link in the article to contact Apple to discuss repair options.