"The staff is working."
It's correct (apart from the missing tail from the last letter, of course), it means almost the same, but this sentence aims to teach you the word 'personel' for 'staff', and I suppose we should stick to that.
The correct solution on Duo is in singular. My answer was: "Personel pracuja" (sorry, the last letter is also wrong). Is the word "personel" singular and also plural?