"My co-workers are great."
Translation:Meine Mitarbeiter sind toll.
Why did it not accept "Mitarbeitern," and then tell me that I should have used the singular?
It did not accept "Mitarbeitern" as it is the dative case form (plural). However, you must use a plural form here of course, not a singular one (as the English sentence has "co-workers", plural). Thus, it should be "Mitarbeiter".