"I am not chatting inside, but outside."
Translation:Én nem bent beszélgetek, hanem kint.
Because "van" is a verb itself, the verb "to be" in the third person singular ("is").
What you proposed would literally mean "I am not chatting; inside he is, but outside (he is)". (or something similar, may the native speakers correct me, but anyway, a sentence like that would make no sense)