In English questions start with a verb. If we remove the question mark from the Ukranian sentence, the correct English translation would be "You will be home tomorrow."
Technically you are right, it's a very common and correct way to ask. Like asking "You want some tea?" rather than "Do you want some tea?"
I added it as a correct version, but I have a suspicion that it might be that the general agreement among the Duolingo team is not to accept this input because Duo ignores punctuation. and this way it's not clear whether the learner meant such question, or he simply did not understand the task and wrote a statement instead of a question...