Update issues are really tricky. Usually it's causes by a mismatch of location and region things.
Eg, someone buys a phone in say Spain, flies to the uk and lives there, updates break.
But with you that probably isn't the case. While one ui 8 isn't out yet, you are certainly behind on security patches by a few months.
The nuclear option that should 99% work is wiping the phone and reinstalling android using a computer. But that's very extreme.
As of the moment, I can't really think of much else you could try. See if anyone else comes in with an idea