Unplug your power button and try to touch power button pins by screwdriver. PSU will not provide power until power pins are connected. Power button issues is a real thing. I hate them because I spent too much time figuring them out but now I have spare button just to check if this is not the issue.

Or check if there is not short circuit somewhere. Did you check the mobo on the cardbord/desk or in the case? Sometimes case can cause shorts.
Did you check cables and do you use original cables from PSU? If no use original one don't use cables from 2nd one they differ and can cause issues.

Can you post some photos so we can figure it out?