"Yesterday, my father assembled a cabinet."
Translation:wa'Hu' DeSwar mutlh vavwI'.
-pu' is actually correct here. The speaker is speaking from a standpoint of the cabinet being completed, and when something is being described as completed, you need to add one of the perfective markers. Lacking -pu' is actually incorrect, or else it takes a stance that the English doesn't: that the speaker is adopting a viewpoint back in time, yesterday, when his/her father was assembling a cabinet.
But the creators of the Duolingo course have decided that, since Klingon perfective doesn't exactly match anything in English, they're going to artificially require that Klingon perfective maps exactly onto English perfect tenses (have done, had done, will have done), and anything else that should have it will simply not have it. It's not a design decision I agree with.
wa'Hu' DeSwar mutlhpu' vavwI'
Yesterday my father assembled a cabinet.
Yesterday my father had assembled a cabinet.
(Either of these are good English translations.)
wa'Hu' DeSwar mutlh vavwI'
Yesterday my father assembles a cabinet. (Describes a thing that is not complete. Might be used if I'm telling a story putting the listener in the middle of the action.)
According to what you have just said, then, the translation given above should be Translation:wa'Hu' DeSwar mutlhpu' vavwI' - i.e. it's the translation that needs to be fixed and not the option in the question. Still, I have understood the reasoning of the persons who put together the course. I cannot agree or disagree with their choices because I am a novice in Klingon; however, I just thought they ought to be consistent with what has been presented thus far, in which case the -pu' should be removed from mutlh in the "correct" option presented with the question, IMO.