My FACE ID isn’t working an my phone is still under warranty

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

What was the result when you followed the guidance in the Related Article above your name on this page, that you were reading just before you posted here? If the Related Article doesn't answer your question please post back with more details about the issue and another user in this user-to-user forum will try to help.

You will note that the last step in that article is a link to contact Apple support to arrange a repair. If the phone is in warranty, as you state, the repair should be free.