A piss-up requires alcoholic beverages and people to drink them. The beverages would be readily available at a brewery, so the only planning task required would be how to get people there. Someone who cannot complete this simple task successfully is presumed to have trouble planning for a more complex project.
couldn't organise a piss-up in a brewery
- (UK, Australia, New Zealand, vulgar, slang, idiomatic) Is unable to successfully plan a simple project; is incompetent.
- couldn't organise a two-ticket raffle
- couldn't organise an orgy in a brothel