"Come down quickly from the mountain" is also correct and should be accepted as a correct answer.
Please come down (from) the mountain quickly
Please come quickly down the mountain
These should all work
Is there a reason why "Quickly come down from the mountain." wouldn't be accepted?
The commenters properly alert Duolingo to another annoying shortcoming in its development: a failure to allow for English variation in adverbial expressions. Are the developers just weak in English?