Unless you are already on 10.13.5 then you cannot update using the regular delta updater, you must use the combo updater, which will work for anything from 10.13.0 up to 10.13.5
Link is here Download macOS High Sierra 10.13.6 Combo Update
After [many] comments:-
If this doesn't work for you... then assume your own situation is not similar, not that the answer is wrong.