。
Don't forget to fasten your seatbelt.
Literal
Not-forget-[without-ず] safety-belt [object-を] close-[soft-command-なさい]
Two small oddities. 安全ベルト is a literal calque of 'safety belt' and uses ベルト as a loanword — the more common modern term is シートベルト. More notably, the verb is written with 閉める (to close) rather than the correct 締める (to fasten/tighten) — both read しめる, but only the latter is used for belts. Likely a homophone typo in the source.