Use ～てからでないと to state that some action is a prerequisite: without doing A first, B is impossible, difficult, or unwise. The second clause must be negative or express inability/difficulty (できません, 難しい, わかりかねます), so it is common in business for deferring decisions, declining on-the-spot answers, and setting conditions in negotiation. In writing and formal speech, ～てからでなければ is the stiffer variant; ～てからじゃないと is the casual spoken one.