"I am going to Paris tomorrow."
Translation:Je vais à Paris demain.
Do you mean is it possible to just say Je vais aller à Paris ? If so, no, as Je vais aller à Paris = "I am going to go to Paris" but it doesn't state when. Today, next week, next month ...? "Tomorrow" is specified in the given sentence so you need to specify demain.
Well, I just tried "Je vais aller à Paris demain", and it was disallowed. I reported it, and I doubt it will be accepted as a valid translation.
Given how I suspect the DL conventions/style-guide goes, "Je vais aller à Paris demain" would become "I am going to go to Paris tomorrow".
If you omit the "demain" that was asked by this specific exercise, yes it's a valid sentence