"Не зараз, а пізніше."
Translation:Not now but later.
There are certain patterns to usage of conjunctions, and this isn't the one where you would want to use «але».
Structures like "I am doing A and you are doing B" or "it is A, not B" routinely use "а".
Yeah, this can be tricky for English speakers to sort out. If there isn't already a unit on conjunctions, it would be a cool addition.