"At nine I am back again."
Translation:Um neun bin ich wieder zurück.
"Um neun komme ich wieder" is correct, although, to be exact, it's really "I (will) return at nine", while Duolingo's version sticks to a more literal translation.
Maybe Duo rates it as a mistake because if you stick with "I am --> ich bin", you do need the "zurück" and Duo can't differentiate there. ("Um neun bin ich wieder" would be wrong.)
All in all, I'm not sure if Duo should accept it or not (because they usually want more literal translations), but in real life, "Um neun komme ich wieder" is perfectly fine and probably preferred over "... zurück", since the "zurück" isn't necessary.
(In real life, you can also say, "Um neun bin ich wieder da", which would probably be my preferred version (telling it to my family when I leave the house - "I'll be back at 9"), along with "Ich komme um neun wieder" (telling it to a salesperson when I'll pick up a product later in the day - "I'll return here at 9").)