不得 is formal and often found in rules, regulations, and written notices. In everyday speech, 不能 (bù néng) or 不可以 (bù kěyǐ) is more natural.
grammar
不得 can also appear after a verb as a complement meaning "cannot" (e.g., 了不得 liǎobùdé "extraordinary/terrible"). The standalone 不得 here means "must not."
