[ابو بيشو]

when unless cannot be used
Unless (= 'except if') can be used instead of if not when we refer to
exceptional circumstances which would change a situation (see above
examples). We do not use unless to refer to something negative that would be the main cause of the situation that we are talking about.
My wife will be very upset if I don't get back tomorrow.
(N 0 T My wife will be very upset unless I get back tomorrow.
If the speaker does not get back, this will be the main cause of his
wife's unhappiness — if not doesn't mean 'except if' here.)
She'd look nicer if she didn't wear so much make-up.
(NOT She'd look nicer unless she wore so much make up. If not doesn't mean 'except if' here.)
I'll be surprised if he doesn't have an accident soon.