I thought this answer might be correct, but I stand to be corrected: "What are you having an argument about this time?"
It was a missing answer. Please use the report function next time if you feel your answer should be accepted.
About what do you all fight this time? - is indicated as correct answer. Where is "all" in the Dutch sentence??
We accept "you all" as a translation of "jullie", as it is commonly used that way in parts of the English-speaking world. Of course it's not necessary to include it. Who knows why Duolingo decided to suggest it to you, it works in mysterious ways.