iPhone XR unable to use Face ID: How to resolve the issue?

Work down this list:

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

It’s likely that it’s a hardware issue in one — or more — of the components used for FaceID.

If so, after Step 9 you might determine that a replacement phone is the most cost-effective option.

You have a pretty old iPhone at this point and the True Depth Camera could have simply failed. See this link for help, but if nothing works, you'll need to schedule an appointment at your Apple Store Genius Bar for service options --> If Face ID isn't working on your iPhone or iPad Pro - Apple Support

If this is a hardware issue, which is likely, it will require having the screen on your iPhone replaced. Personally, I wouldn't invest any money in a phone from 2018, 7 years ago. iPhone Xr cannot be updated past iOS 18.7.5 at this point.