As opposed to "no" or "not now" you can also say "wait a moment" or "I'll help you later". No need to explain. Just make sure there's a way forward. She just needs to hold on a bit. Giving "no" even with an explanation as to why not might not yet show a way forward.

My daughter always comes with all kinds of questions when it's bedtime (and she doesn't want to sleep). Simply answer: "Let's talk about that tomorrow. Now it's time to sleep."