。
Don't leave the bathroom a mess.
Literal
Bathroom [object-を] scatter-[leaving-in-state-っぱなし]-to do-[negative-ないで].
~っぱなし is a colloquial suffix meaning 'leaving X in an uncompleted/undone state' — 散らかしっぱなし = 'leaving [it] scattered / messy.' Has a slightly reproachful tone. ~にする ('make it X') combined with the imperative ~ないで ('don't') = 'don't leave it [in that state].'