That means that you're on the roof. We can't accept that, because many Polish people take this course as a reverse tree and they can try to answer like that.
that's a very absurd reasoning :/ why does it matter what the reverse tree takers would do if the tree isn't even meant for them
How about this reasoning: any native speaker of English would consider "on the stadium" wrong unless there was a very clear context stating that you mean on the roof of the stadium. Seriously, this is a learning website, we can't accept stuff like that.
What does the roof have to do with this? Most stadiums are pretty flat pieces of land where some play sports and others may be watching. No roofs involved. Being ON the stadium makes perfect sense, especially if you're a player.
But I think stadiums are still seen as closed buildings from a grammatical point of view. Would you really say a player was on the stadium? On the pitch, on the track, on the court etc. sure, but on the stadium?
"On" the stadium doesn't sound right at all, at least to me (native U.K.). A stadium is the whole structure surrounding the playing area.
Metallica might be on at the stadium, but that's the only context for "on" I can think of, apart from, say, a construction worker working on the stadium, (i.e., building or repairing the stadium)