〜てからでないと／てからでなければ
Can't… unless… firstmachine translation
FormationVerb て-form + からでないと／からでなければ + negative clause
〜てからでないと expresses "unless A is done first, B cannot be done (is not allowed)": 予約してからでないと入れません (you can't get in unless you've made a reservation first). A is a prerequisite that must be completed beforehand.
The point to remember: the second clause is ALWAYS negative in form or in meaning — できません, 無理です, だめです. The form 〜てからでなければ is slightly more formal, but the meaning is unchanged. You can see traces of the と・ば conditionals you learned at N4 in the very structure of the pattern: "if it is not (the state of) having done A, then…".
Compared with 〜てから (N5), which just states the sequence "do A, then do B," てからでないと stresses that A is a prerequisite — without it, B is impossible. Meanwhile 〜て初めて looks back at the past ("thanks to A, B happened for the first time"), whereas てからでないと points at what hasn't happened yet and is often used to refuse or defer politely: 上司に聞いてからでないと、お答えできません.
Examples
予約してからでないと、その店には入れません。
You can't get into that restaurant unless you've made a reservation first.
親と相談してからでなければ、決められません。
I can't decide until I've talked it over with my parents.
手を洗ってからでないと、ご飯を食べてはいけませんよ。
You're not allowed to eat until you've washed your hands, you know.