Translation:Did the guard assist you? No. She didn't assist us.
In sentences like this, how can you tell what gender is being used without actually seeing the person?
Because Klingon differentiates singular from plural and you are in a learning situation, we do not allow the use of the singular "they" in this course, so the software can tell whether you mean singular or plural. If you use the singular "guard" in your English translation, you must use either "he" or "she" in the response.
However, since plural markers are not required in Klingon it IS possible to translate the Klingon sentence with plural subjects. "Did the guards assist you? No. They didn't asssist us," is also an accepted answer. But you must put the -s on "guards" to match the "they" in the response.
Outside Duolingo, if you translate ghaH as they, in context, there's nothing wrong with that, and if you don't know a person's English pronoun, they is probably the best choice, but for the purpose of teaching all the prefixes on this course we're pretending they can only be 3rd person plural.
Whenever a translation is ambiguous, e.g. ghaH can be he or she and you see can be bIlegh or Sulegh, just pick one of the possibilities. If it's not accepted, and you've double-checked your answer for typos, report it with the "My answer should have been accepted" option.