Face ID is not working on iPhone

Start working your way down this list;

If Face ID isn't working on your iPhone or iPad Pro - Apple Support

Note that the list “ends” with you having the phone assessed by a service technician; and a possible repair or replacement bill.

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If Face ID isn't working on your iPhone or iPad Pro - Apple Support

Note you may click o other highlighted tabs of interest for further assistance once inside the Apple link above.

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There may be something wrong with your TrueDepth camera. Please view the following articles. 

If Face ID isn't working on your iPhone or iPad Pro - Apple Support

If an alert says Face ID has an issue on your iPhone or iPad Pro - Apple Support

If the above doesn’t help, please make a reservation at your nearest Apple Store Genius Bar or Apple Authorized Service Provider for diagnostics. 

Genius Bar Reservation and Apple Support Options - Apple