a problem was detected with the truedepth camera. face id has been disabled

if it was me then I would take a backup

and then try to restore it to factory settings Restore your iPhone, iPad, or iPod to factory settings - Apple Support

if it was still a problem then I would expect it to require repair to work once more

He should go to apple store to get him repair face id

You should never update to iOS 15 Beta if you aren’t a developer. But you found a bug yaaay.