Face ID not available

Review the information here:

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

The 2 most common reasons where you would see an error message are:

• A case covering a portion of the front facing camera module
• A hardware failure in one of the sensors of the camera module. The Support Article above also includes a link to schedule service for diagnostics and repair.

That is almost always a hardware problem.

Make an appointment at the Genius Bar of your local Apple Store or take it to an authorized service provider to have it checked.