that ... isn't allowed걸 is a shortened form of 것을, which turns the preceding clause into “that …” or “the fact that ….” 안 되다 can mean that something is not allowed, not okay, or not possible, depending on context.
KO: 여기서 사진을 찍으면 안 된다는 걸 몰랐어요.
EN: I didn't know that taking photos here wasn't allowed.