Unable to change Face ID on iPhone

There may be something wrong with your TrueDepth camera. Please review the following support 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 Genius Bar or Apple Authorized Service Provider for diagnostics. 

Genius Bar Reservation and Apple Support Options - Apple 

Perhaps this article will help: Face ID not working.