"내일은 여자친구를 볼 거다."
Translation:I will see my girlfriend tomorrow.
My response "Tomorrow i'm going to see my girlfriend." and Durolingo's response "Tomorrow, I will see my girlfriend." mean the same thing. Yes, I reported it.
I think Duolingo is trying to make a distinction between "I will do x" and "I'm going to do x," even though we use them interchangeably in English.