Face ID not available on iPhone

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 or Apple Authorized Service Provider for diagnostics. ⇢ 

Genius Bar Reservation and Apple Support Options - Apple

That is almost always a hardware failure, but you can try this→If Face ID is disabled on your iPhone or iPad Pro - Apple Support