。
You're not allowed to go out after dinner.
Literal
Dinner-after [topic-は] go-out-[gerund-て]-is-not-allowed-[polite-ません].
A firm parental rule. 外出する ('to go out, leave the house') is a formal-ish suru-verb, slightly more written/formal than 出かける, and often marks leaving the house for reasons other than errands — it's the word used in curfews or public health advisories (外出自粛, 'refrain from going out'). ~てはいけません is the polite prohibitive: 'must not.'