This is not an imperative. Imperatives use a special set of prefixes and must have a prefix. You will learn those later in the course. In fact, this is not a full sentence at all. Notice that there is not a period. This entry is actually looking for the dictionary definition. Though, since jatlh could also be a full Klingon sentence, this entry does also accept full English sentences as translations. However, "speaks" is neither the dictionary definition nor a full English sentence and so is not accepted.
jatlh torgh is "Torg speaks".
And «jIDoy'» jatlh torgh is "Torg says, «I'm tired»."
But as jdmcowan wrote above, jatlh on its own is not a sentence -- it's just a word of its own, a vocabulary word, a dictionary entry. And we're looking for the corresponding dictionary entry in English -- not all the translations that could be used if that word were used in a sentence.
It's not like it's a wrong answer!
Well, yes and no.
In a full sentence, "speaks" can be a good translation of jatlh.
But here, it's not part of a sentence -- it's just a vocabulary word, kind of like a dictionary entry. And most dictionaries would not translate jatlh as "speaks" but rather as "speak" or perhaps "to speak", using a kind of dictionary form of the English verb.
In some cases, treating it as a sentence and translating it as "he speaks, she understands, it explodes" etc. will also be accepted, but the form with -s and without subject is generally not accepted so far.
I'd recommend that you translate bare jatlh as either "speak" or "to speak" rather than "speaks", at least here on the course.
None of what you said hasn't already appeared in this discussion. Don't you think it's a little beside the point of the exercise to expect the student to come to the conclusion that some form of the verb (without an -s) is better as an answer because there's no punctuation in the sentence to be translated? ...especially when there is another very good Klingon language course that in fact gives "speaks" with the -s as the translation of "jatlh".
Personally I think that other course is wrong to do so and I'm sorry to hear it has misled you. We have expressed our reasons for sticking to the translations we have already provided. I hope you can continue to enjoy this course despite the differences from a different course with which you are already familiar.