Is there a proverb or idiom describing incompetence?

Solution 1:

Not know your arse from your elbow — Cambridge

offensive To be stupid and unable to understand very simple things

Not be able to do something to save your life — Macmillan

to be very bad at doing something
"She couldn’t play the piano to save her life!"

Solution 2:

Couldn't organize a piss-up in a brewery — Cambridge

UK, offensive Said about someone who is completely unable to organize things

Solution 3:

Couldn't pour water from a boot (if the instructions were on the heel) - TheFreeDictionary

Stupid.

Anecdotally, I've also heard piss instead of water, if you're feeling somewhat more vulgar.