"When the show starts?"
The English sentence is slightly off, it should be "When does the show start" - reported
A fair number of Duolingo's "sentences" are really just fragments. I came across a good one the other day: "Except for the shoes" (huh?). So this could be a fragment of a two sentence question: "When can I sit down? When the show starts?" or a longer question "Can I sit down when the show starts?". I'm sure you are right to report it though.