Hello, why can't I use Face ID?

Have a look at these two articles -

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

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.

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

if it say it's disabled then have you read If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

and maybe reached out to their support Apple Support

?