Couldn't train a choko vine to grow up a dunny wall

Australian descriptive metaphor.

The person in question is of such incompetence or stupidity they indeed couldn't train a choko vine to grow up a dunny wall. Commonly used to refer to a dim workmate or acquaintance.

The choko (also known as a cheyote or chouchou, among others) being an extremely hardy vine fruit that, left to its own devices, will grow up virtually any surface.

The paperwork's wrong again! Honestly, that moron couldn't train a choko vine to grow up a dunny wall!