Because it is something that has masculine gender
"Nicht einer (Spiegel), sondern zwei?"
In German "eins" is the number only, "einer" is an indefinite pronoun that takes the place of a noun (here a masculine noun). http://www.canoo.net/inflection/einer:Pron:SG:Indef
So, you would count "eins, zwei, drei..." http://german.about.com/library/blmus_kinderPolizei.htm
If you are talking about a thing, but you need more than one, you could say "Nicht einer, sondern zwei".
There is also a noun form "der Einer". http://www.canoo.net/inflection/einer:N:M
Compare to the indefinite article (a, an): http://www.canoo.net/inflection/ein:Art:Indef:SG
"Not one, rather two" is wrong, but it suggested "not one, but rather two." Don't "rather _" and "but rather _" mean the same thing?
Did you try "eine"? I think it depends on the previous sentences. Without context, shouldn't all three gender possibilities be accepted?
There does seem to be a bit of gender bias towards masculine when unknown: https://en.wikipedia.org/wiki/Gender_neutrality_in_languages_with_grammatical_gender#German
Why is 'sondern' used here in this context, could 'aber' be used to the same extent?
I found that to be a helpful explanation. Basically sondern is used when you're saying 'not this but that' and aber is more for when you use 'but' like 'however.'
I think refusing to accept "Not one, rather two"... while accepting "Not one, but rather two" is silly. I'm native English speaker and in English those two sentences could well mean essentially the same thing. The requirement of "but" just seems silly.
OnkelD, I have submitted many similar comments to Duolingo in their "Report a problem" section. My comments there were not shared with the membership or fellow students. I can see that you have a higher multi-language capacity thank I do, congratulations on that. By your comment here, I can also see that you share my annoyance with Duolingo's seemingly capricious non-acceptance of sensible translations. Thanks.