My 13 mini's back glass has a mark akin to a fingerprint. It goes away when scratched with a fingernail. However, it's...

If, by "not working" you mean that the customer gets the message "Unable to activate Face ID/Touch ID on this iPhone," then it is because the Face ID and Touch ID components are serialized to the logic board. They cannot be replaced. Apple cites security concerns for this, but I think it is just that Apple wants you to have one more reason to buy the latest iPhone instead of repairing your old one. #rightrorepair Hope this helps!