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
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.'
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.