Duolingo is just picking the word closest to the word you typed. Y'all has two l's at the end, like will. The first bit of the sentence is an address, and the last bit is the main sentence.
Also, take a look it at the prefix, Su-. That's the second person plural subject/no object prefix, so it has to be "you (guys) will jump" or "you (guys) jump" or "you (guys) jumped".
We are teaching Klingon, not English. If someone enters "y'all" as part of the translation and uses it to show that they correctly understand the Klingon, then we should accept it. Unfortunately, Duolingo will occasionally pull those sentences out to show you. There may be a couple sentences where "y'all" was actually entered as a possible translation and as we find them, we'll remove that. Hopefully the contraction of "you all" will continue to be accepeted, but not shown as a possability. There shouldn't be any more problems with this sentence.
English has many ways to express second person plural, depending on the area :)
As a general rule, this course accepts "you", "you all", and "you guys" as second-person-plural pronouns, and the system automatically lets you contract "you all" to "y'all".
So "you all will jump" is accepted.
I can add "you will both jump" to this sentence, but in general, if you want to use "all", your best bet is to put it right after you" rather than after an auxiliary such as "will".
You may report additional combinations if you wish on individual sentences, but it may be easiest for everyone if learners adapt to the Procrustean course rather than the course contributors modifying thousands of sentences to add an additional second-person-plural pronoun.
Because that's how Klingon does it. Noun conjunctions come after the list of nouns. In English we put it before the last one (which seems natural to me as an English speaker, but illogical if you really think about it), but in Klingon it always comes last. There are also sentence conjunctions and they come between each sentence.