You must fasten your seatbelt during takeoff.

Literal

Takeoff [possessive-の] occasion [topic-は], seatbelt [object-を] must-fasten.

離陸の際は (during takeoff) uses 際 (さい, occasion/at the time of) — a formal time expression. シートベルトを締める (to fasten a seatbelt) is standard vocabulary. ~なくてはならない is the obligation pattern.