My Face ID isn’t working and system showed it is unavailable at this moment

If Face ID isn't working for you, there is most likely something wrong with your TrueDepth camera. First, look at these two articles for help. If that doesn't fix the issue, then make an appointment at the Genius Bar to have your camera issue diagnosed.

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

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

Make an appointment at the Genius Bar to have it diagnosed.

Genius Bar Reservation and Apple Support Options - Apple

First restart your phone or try face id if this not work then go to apple official care.