How to enable face id

This is generally a hardware issue. If the unit's on an OS version older than 16.5, you might be able to fix it by updating it from a computer or the Software Update function in Settings; otherwise, contact Apple Support.

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If you followed all of the advice in the Related Article Link at the top of this page and nothing worked, then you do what is step #3, which is detailed here:

1. If you still can't enroll your face or unlock your device with Face ID, you can take your device to an Apple Store or Apple Authorized Service Provider, or contact Apple Support about your iPhone or your iPad.