I don't see why "No one is working" is not a proper translation - many of the translations I've given are completely fine but it seems Duolingo is not yet equipt to accept differences in translations.
Wouldnt "working" be 일하고 있습니다? So "nobody is working" should be 아무도 안 일하고 있습니다 :) cmiiw
Even though 아무도 is usually translated as 'nobody' it still needs another negative word in the sentence. So it could also be translated as 'anyone' but it can't be used in a positive sentence.
This double negative business tripped me up in Spanish. At first it felt weird but when you think about it maybe English is weird for allowing double negatives to be a positive...