〜てからでないと
not until, unless (you) first
Verb て-form + からでないと / からでなければ + negative
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てからでないと says one thing has to happen before another is possible: 手を洗ってからでないと、食べてはいけません, "you can't eat until you've washed your hands".
The second part is always negative or a prohibition: できない, 決められない, 入れません, てはいけない.
It's てから ("after") plus でないと ("if it's not"), so the literal sense is "if it's not after…". てからでなければ is a slightly more formal version; in speech, it's からじゃないと.
Compare てはじめて, which looks back ("I only realised after"). てからでないと looks forward and sets a condition, which is why it's so common in rules, instructions and polite refusals.
Examples
手を洗ってからでないと、食べてはいけません。
You can't eat until you've washed your hands.
実物を見てからでないと、買うかどうか決められない。
I can't decide whether to buy it until I've seen the real thing.
家族と相談してからでないと、返事できません。
I can't give you an answer until I've talked it over with my family.
二十歳になってからでないと、お酒は飲めない。
You can't drink until you're twenty.
予約してからでないと、入れません。
You can't get in without booking first.