I can’t use the Face ID. ?

If you see a 'Face ID disabled' message, this document deals with that specific issue:

"If an alert says Face ID has been disabled on your iPhone or iPad Pro", click here --> https://support.apple.com/HT208186

If you just have issues with Face ID not working properly, read this document:

'If Face ID isn't working on your iPhone or iPad Pro', click here --> https://support.apple.com/HT208114

See if anything here helps 

Face ID not working.   -

Face ID not working' problems on iPhone.    -

Fix iPhone X Face ID Not Working.    -

Face ID Not Working.   -