Translation:I do not have an older sister but an older brother.
This is not translation. It is "type what you hear". I had the same problem with duolingo not accepting ağabey for "big brother" in another exercise and was told by another moderator that duolingo accepts "ağabey." Clearly it still doesn't and so duolingo needs to correct this basic mistake of theirs.
"Yapacağım" is not the standard way to say the word. In TV-Turkish, "yapıcaam" is the only correct way to say it, and that's how anchormen/women are taught. "Yapacağım" is to be avoided (although, I do say 'yapacağım' myself, as my speech is not standard). The same thing goes for "abi". Saying /Ağabey/ is not standard Turkish anymore, but the spelling exists. Turkish is no longer a phonetic language sadly.
I actually am team Selcen on this one. I have hear people use both (and I tend to see people from places with more conservative dialects use "ağabey:). Almost everyone I know (with the exception being a large group of host family members from around a village in Polatlı) writes and says "abi."
Also, "yapıcam" is slang=y in Turkish and is definitely not standard according to anyone.
Is abi/abla necessarily "older" sibling? I think in other questions simply brother/sister was accepted. In the "translate by picking words" the choice was not available because there was no "a", just "an", but would it be OK? If not, how would you refer to a sibling not considering the relative sge?