which one is correct

varför gör du det nu?

varför gör du nu det?

See a translation

varför gör du det nu?

I think both sounds fine, but they need context to make sense. Also they differ slightly in meaning.

For example:

Person A: Nu är det dags att köpa julklappar! (Now it's time to buy Christmas presents!)
Person B: Varför gör du det nu? (Why are you doing it now? (indicating that it is weird in some way right now, and that it would have been better to do it earlier or later. In this case, it's clear that B is surprised that A buys the presents so late, since it's Christmas tomorrow.))

And:

Person A: Nu är det dags att köpa julklappar!
Person B: Jaha, ska du köpa julklappar! Hmm, bara en fråga, men varför gör du nu det förresten? (Oh, you're going to buy Christmas presents! Hmm, by the way, I just want to ask but why do you do that again? (This indicates that B has no idea why you should by Christmas presemts at all))

Note also that this kind of "nu" can be used in other situations to express uncertainty, for example:

Vad var det nu du hette? = What was your name again? (Indicates that you try to remember the name, that you usually SHOULD remember it, but for some reason forgot. So it's a pretty polite way of asking a persons name that you forgot.)

I'm just speaking from my intuition, but I hope it helps!