"Go back to sleep, Torg!"
There doesn't need to be, but to be consistent we should have one there. There is no possability of confusion that Torg is the grammatical subject since Klingon imperatives can't have a third person subject. Torg can only be added on as the name of the person being addressed in this case. If his name were called at the beginning of the sentence, it would be a bigger issue, since an imperative with a yI- prefix could theoretically have an object (though I don't know what it would mean to "sleep Torg again"). The Duolingo system makes it a hassle to redo the whole sentence just to add a comma. We might eventually get around to doing it, but since there is no chance of confusion in this sentence, it's going to have a low priority.