"What juice do you want?"
Seems to me 你想喝什么汁 should be accepted. The 要 isn't necessary and the prompt didn't specify fruit juice -- though, admittedly, one would assume it.
I agree that 要 doesn’t seem to be necessary here.
你想什么果汁？is what I tried (marked wrong).
But, on reflection, it does sound nicer with the double syllable. So I’ll try to learn it that way.
I used 你想要什么汁？and was marked incorrect. Isn't 汁 by itself indicative of 'juice'? The question didn't specify 'fruit juice'.
By itself 汁 means "juice" in the sence of "fluid" or "sauce". When meaning a fruit juice it's either 果汁 or the specific juice 苹果汁, 橙汁 etc. (I don't think I've ever heard 汁 alone in this context) But in English it's normally shortened to simply "juice". Admittedly it takes some "reading between the lines" but that's language learning.
Just to explain, the 什么 is "what" and the 果汁 is "fruit juice". So if you put the "what" at the end the syntax is wrong.