。
Don't lean against the wall.
Literal
Wall [against-に] lean-against [negative imperative-な].
もたれかかる (凭れ掛かる) is a compound combining もたれる ('lean') and かかる ('hang/rest on') for emphasis — a slightly more vivid 'lean fully against, slump against' than plain もたれる. The negative imperative ~な makes it a curt prohibition. Japanese has multiple verbs for leaning (より掛かる, もたれる, もたれかかる) that differ in register and intensity, useful for nuancing physical descriptions.