Because it wasn't in the list of accepted answers apparently but according to the comment of the Mod below, it is now.

Joues-tu avec nous ? is the way to write this in the "tu" form. Remember that French doesn't need an auxiliary verb to form a question like we do in English. (Additionally êtes is the conjugated verb for vous).

Ex. In English to ask someone if they like the music, we write: Do you like the music ?

In French to ask the same question: Aimes-tu (Aimez-vous) la musique ? Note that there is no equivalent to "do" and no auxiliary verb is used. Nor is the word are used in the lesson question.

If you want to switch it around you can also write:

Est-ce que tu joues avec nous ? / Tu joues avec nous ?

That is correct and should have been accepted. For some reason it was missing, but it is there now. Please allow up to a week before for the system to make the updates and recognize it as correct. Report it in the lesson if it is still counted wrong after MAY 30th 2018. Thanks!!