Why can’t i turn on face id ?

Hello,

Apple has several documents helping you with Face ID.

How to set up and use Face ID is described in this document --> Use Face ID on your iPhone or iPad Pro - Apple Support

- including how to turn on

This document is specific and explains what to do if you see a 'disabled' Face ID alert --> If an alert says Face ID has been disabled on your iPhone or iPad Pro - Apple Support

If you don't see a 'disabled' message but do have general issues with Face ID not working properly, explore the list of things to try in this document: If Face ID isn't working on your iPhone or iPad Pro - Apple Support

If the first 8 don't solve the issue then use #9, contact Apple.