。
You must not get your priorities mixed up.
Literal
Main-and-end [object-を] reverse do [must not-てはいけない].
本末転倒 ('putting the cart before the horse,' literally 'main and end reversed') is a 四字熟語 warning against confusing essentials with trivialities. Pairing it with ~てはいけない ('must not') gives a stern warning rather than a barked prohibition — the construction softens the imperative into a moral injunction often heard in advice and editorials.