"What do you want to know?"
Translation:Qu'est-ce que tu veux savoir ?
With the inverion that would be: "Que veux-tu savoir ?"
Putting que/quoi at the end of the sentence is a colloquial wording, and it cannot be combined with the very formal inversion: "Tu veux savoir quoi ?"
However I wouldn't recommend using colloquial constructions on duolingo, and you should stick to the more standard constructions: "Qu'est-ce que tu veux savoir ?" and "Que veux-tu savoir ?"