Why my Face ID is not available for my iPhone now?

If you're seeing the alert that is discussed in the Related Article that you came here from..

Then you need to follow the instructions provided:

1. Update your iPhone or iPad to the latest version of iOS or iPadOS.
2. If you still can't enroll your face or unlock your device with Face ID, take your device to an Apple Store or Apple Authorized Service Provider, or contact Apple Support about your iPhone.

For #2, when you contact Apple Support, do not select 'get help from users around the world'.. As the article says, you need to contact Apple, not other iPhone users.