Face ID doesn’t work on my iPhone and I don’t know why?

See Set up Face ID on iPhone https://support.apple.com/guide/iphone/set-up-face-id-iph6d162927a/ios

And see If Face ID isn't working on your iPhone or iPad Pro https://support.apple.com/en-us/HT208114

If after following the first seven steps given, "... you still need help, reset Face ID, then set it up again. Go to Settings > Face ID & Passcode and tap Reset Face ID. Then tap Set up Face ID to set it up again. 

If you can’t enroll your face, take your device to an Apple Retail Store or Apple Authorized Service Provider or contact Apple Support." You'll need to contact Apple Support at this time unless an Apple Store is open in your area.

See if anything here helps.

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