I answered "May you not come" and it was also marked wrong. The interesting thing here (at least I feel it interesting) is: either "Can you not come?" or "May you not come?" has different meaning from the suggested answer "Are you not allowed to come?"
So perhaps our answers are really incorrect. But I am a little bit confused with this now....
Ah, I would expect the English sentence to be "are you allowed not to come".
In Dutch it might be easier to ask the opposite: "mogen jullie wegblijven?" (are you allowed to stay away). Otherwise you could say "is het jullie toegestaan om niet te komen?" (is it permitted to you not to come).