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See this support article:--> Use Face ID on your iPhone or iPad Pro – Apple Support
and/or this:--> If Face ID isn't working on your iPhone or iPad Pro – Apple Support
and/or this:--> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support
If the issue persists, contact Apple Support directly, using this link:--> https://getsupport.apple.com/?caller=kbase&SG=SG003&category_id=SC0999&symptom_id=23303 , or make an appointment and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.
Regards
Giulio