Translation:He will sleep.
The final period/full stop in Qong. shows that this is a sentence.
Qong. is a complete sentence in Klingon.
"Sleep." is not a complete sentence in English.
If the prompt had been Qong (without the full stop at the end), then "sleep" would have been accepted -- then it would have been asking after a Klingon dictionary word, and an English dictionary word would have been accepted.
In English, you cannot omit the subject.
So you have to pick some subject.
Either "He sleeps" or "She sleeps" or "They sleep" -- theoretically even "It sleeps" (perhaps an animal).
Which one you pick is up to you; the Klingon doesn't give context here.
But you do have to pick one, or it's not a valid English sentence.
English is being used as the mode of instruction. If you want the course to include every single ungrammatical sentence possible in English, that would be infinite and therefore would be an impossible course to make. By limiting the answer translations to grammatical English and to some reasonable rules that conform to proper English spelling and syntax, it becomes actually possible to create this course.
You can translate Qong. as "He will sleep."
You can also translate it as "She will sleep."
What does not work is if you translate it as "He will sleep., She will sleep."
Those are two English sentences to translate the one Klingon sentence -- pick one or the other but don't write both at once.
If you got those as a correction, it should say something like "Correct answers are" (i.e. plural). rather than "The correct answer is".
Two sentences will definitely be rejected. You must choose one translation to answer with. But if the sentences were rejected individually, then that must be some software error. If it happens again please post a picture online and post a link to the picture in the sentence discussion.