"Do you think tomorrow will be a sunny day?"
I wrote that too. And i think it's in the same sense correct to write that but its meaning is slightly different. It would mean 'do you think tomorrow's weather will be fine/sunny?'. In some questions on duolingo these are technicalities and are accepted as right and in others it isn't accepted... All I can say duolingo is only really useful as a support aid to proper chinese language classes, by itself it's not that great and this question shows that.