。
Don't lean on the wall.
Literal
Wall [against-に] lean [negative imperative-な].
もたれる is the everyday verb for 'to lean against,' a near-synonym of より掛かる but slightly more common in conversation. The negative imperative ~な appended to the dictionary form gives a curt 'don't X' — typical for posted warnings or sharp parental directives. Japanese has several near-synonyms for leaning (より掛かる, もたれる, もたれかかる) with subtle differences in register.