That would be "Я хочу обміняти гроші". I think this exercise wants to test that you learned the meaning of "валюта".
money == гроші
My question is why not "the currency"? If we are at the currency exchange we want only one specific currency not whatever.
"I want to exchange the currency" is accepted too.
If your answer wasn't accepted, it happened for some other reason. I cannot tell you for what reason because you didn't provide your answer here. Please always provide your full answer in the comment if you have a question about why it wasn't accepted.