Your sentence uses formal Hindi words while the answer given uses informal Hindi words. A native Hindi speaker is much more likely to speak the sentence written in the official answer than the one you wrote.
But obviously both sentences should be added as correct answers.
Is there any pattern to the placement of 'nahi'? It seems to be either at the very end, or second in place, with or without hai. I'm pretty sure this is discussed elsewhere but it's hard to find on the app version. TIA.