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!

vine up

To smother someone/something by wrapping tightly around them/it, like vines strangling a tree slowly as they climb.

Vine up: Watch out, Jesse's gonna vine you up like she did Sonia before they got out untethered by kids and unsmothered by marriage.
Or,
He vined up that tree like English ivy, promotion after promotion, until he sucked the life out of his employees, once interns like himself, now his corporate bitches.
Or,
Fuck this noise. I'm out before I get vined.