Duo still rejects "Are you allowed that?" I reported it also. July 28, 2018. This is what happens with a Duo exercise that's been around for only 3 months and not 5 years.
Darfst du das? could mean Are you allowed to do that OR to have that, etc. Since the German sentence doesn't specify, I don't see why the English sentence needs to make a guess, although I suppose "Are you allowed that?" would be understood as "Are you allowed to have that?", while "to do that?" is probably more common.