"Zij hebben de koffie gemaakt."
Translation:They have made the coffee.
You need to translate "de koffie".
"They made the coffee" is accepted. ;)
I think it is, but Duolingo wants you to do it the long way so that you don't mess it up in the future. Kind of like in Driver's ed you have to do everything a certain way, even though it's not enforced by law.