Well, I think that depends on the seat type. One would sit on a chair, but in an armchair, for example. The most common thing for a fan (οπαδός), to be more specific, which carries a rather negative connotation in Greek, if we translate it that way, would be to sit on a seat, since there's not something surrounding them, more often than not, if I'm not mistaken.
But anyway, I just checked and "in" is already one of the alternatives.
I think I would say that there is more of a tendency to use "in" when the chair is more luxurious, like an armchair or sofa, where there is a space to sit "into". You can't sit "in" a stool for instance.
However in this case, we are not talking about one fan, who (considering the sort of seats at sports venues) would probably sit "on" his/her seat, but rather the mass of fans and the mass of seats. They would sit "in" the seats, because they are amongst them, and not "on" them (altho that might be different for a smaller group of fans/seats).
However, odd if "in" is already there, because I was marked wrong for using it.
What was your exact answer? The "The fans sit in the seats" alternative was missing, I've added it now.
Unfortunately I can't remember precisely - it might indeed have been "sit" rather than "am sitting", which would explain it. Anyway thanks for sorting it out.
Possibly but a better match is seat = θέση (fem.), seats = θέσεις, because a chair is a specific type of seat.
What's going on at the moment? This was marked wrong because I left out an accent - that should be marked as "nearly right". Also the "report a problem" button is offering very few options - I just tried to report a problem but couldn't because it only offered one option, which was not the issue I wanted to report. DuoLingo seems very buggy right now!
Στο καφενείο προφανώς... Γιατί στα γήπεδα και τα στάδια δεν έχω δει καρέκλες... Ποιός τρελός θα έβαζε καρέκλες, που στη διάρκεια του αγώνα θα εκτοξεύονταν στον αγωνιστικό χώρο; :-P