Inna47606, the -un (or -tun) is from Standard Grammar.
A lot of people in their oral conversation don't pronounce the ending sounds (like as the -un/ -tun). So, perhaps its condition is the opposite, ie. it would not sound weird if we don't say the -un/ -tun. Furthermore, maybe some would laugh at us if we spell the -un/ -tun.
However, ... for our learning, we are suggested to pay attention much in Standard Grammar. It will give us many benefits :))
1) "what is that symbol above the l in 2akl that makes it 2aklun"
It's called as "Damma-tain", or literally means two Damma/ a pair of Damma. It's part of ending sounds. Perhaps, in English, it is the resemblance of "diftong".
2) "and in what section of duolingo is it taught?"
As far as I know, Arabic Duolingo contributors don't teach us the ending sounds yet. They focus on the Arabic oral conversation for Foreigners/Travellers. Their method is to omit endings as much as they could, they thought it will save much time in our learning. Many learners don't need to give their time and ask the question about the endings. When I asked about this issue to the moderator, he only said it's okay in the conversation. Still, I, myself, believe the endings are important! :)
I don't know if both are currently accepted by Duolingo because I was using the word bank when this came up, but this really should be Arab, not Arabic, as the English translation of the adjective form. Arabic should be reserved for the language. Arabian (if you were wondering...) refers to the Arabian peninsula. Don't take my word for it...look it up!