I plan on cursing someone in a couple days but the moon is waxing. Will this make my spell unsuccessful?
No, The Moon have an Influence on Magick but not to the extent to make your Ritual unsuccesful if your not doing it during the right moon Phases.
Doing a Curse on a Waning Moon will Increase the Potency of your Curse For Sure But its not mandatory,
If Possible, try doing the Curse while your Target is asleep, 3 am is usually the best time to do a Curse.
The thing to remember with magic is that it’s only influenced by two things 1) your personal power 2) your focus. The reason we do rituals when casting and invoking isn’t because we need to, it’s because it makes it feel more legitimate to us and thus helps us focus better and draw on our own power. So to answer your question, if you feel like it will it will. If you feel like the moon is irrelevant than no it won’t.