My face id can't set up

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

If an alert says Face ID has been disabled on your iPhone or iPad Pro, 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