Right. The captain is listed first in the Klingon and so should also be listed first in the English. I know it may seem excessively strict, but there may be a reason why the Creator of the sentence wanted to list the captain first. And we also want to discourage users from just translating all the parts of the sentence in reverse order.
It also didn't accept "The doctor and the captain were serious." :(
Indeed, because HoD means "captain", not "doctor", and Qel means "doctor", not "captain".
Thus HoD Qel je means "the captain and the doctor".
If you switch them around, it looks as if you're confused about what HoD and Qel mean and that you mixed up their meanings, writing "doctor" for HoD etc. So it marks that option as wrong.