"They are workers."
Translation:Oni są robotnikami.
It will be accepted now.
As Djenthallman wrote it can be either "oni" or "one" and we do omit the 3rd person pronouns a lot less often than the others, but I wouldn't say that they totally "shouldn't" be omitted. It's probably better for the learners not to do it. But after all, in real language you have context, so most probably it would be known who "they" are.