The answer provided by duolingo is a double negative.
Double negatives work differently in Polish than in English.
The translation is correct - 'nikt' requires to negate the verb.
In Polish grammar you can have double, triple, quadruple or even quintuple negation and still it means just negation.
EDIT sorry, this should have been under the PeterChusz's note.
Clear, thank you. I would love to see an example of quintuple negation!
"Nikt nigdzie i nigdy nic nie zrobił w tej sprawie." - Nobody ever and in any place did anything in that matter (Nobody never and nowhere didn't do nothing in that matter)
Thank you! Worth a Lingot (...the best Lingot I ever spent).
Wow. Great example.
Why isn't correct "Tu nikt nie pracuje."?
Correct, added now.
But it can't be "Nikt nie tu pracuje"? Does the "nie" always have to be next to the verb?
It should be next to what it negates, so you actually wrote "No one not here works", which does not make a lot of sense. But "On nie tu pracuje" would be acceptable (He works not here - but somewhere else).
Why not Nikt nie tu pracuje?
Nikt nie tu pracuje?
Because when you negate a verb, you have to put the negation right before the verb: "Nikt tu nie pracuje?"