unable to use face id

Hello~ Click on below and review…

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

Likely you have a hardware issue and need to get your iPhone checked out…

Genius Bar Reservation and Apple Support Options - Apple

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~Katana-San~

Without any additional information on what happens when you try, I could only recommend scheduling an appointment with Apple for diagnostics to be performed. Any failure in the front facing camera module including the True Depth camera or IR blaster will result in a message that says it cannot be used.

https://www.apple.com/retail/geniusbar/

In other cases, which I am not sure this is what you are referring to, a Passcode will be required instead of using Face ID. For example, restarting the phone will prompt for a Passcode instead of Face ID.