More likely a qach mutlhwI', if we look at the example compounds beb mutlhwI' "roofer", majyang mutlhwI' "tiler", and 'och mutlhwI' "plumber".
(Compare, perhaps, also English compounds such as "trouser leg" or "scissor blade" which use the singular "trouser, scissor" in compounds even when they are plural "trousers, scissors" on their own.)
It isn't required and a version without it is listed as an accepted translation. The "best translation" (the one shown to you) includes 'oH to distinguish it from ghaH which could be possible if you don't include a pronoun. But it's just fine without the pronoun.
If your answer was rejected it might have been for a different reason. I don't see any reports. Did you get a screen shot?
Ah, thank you!
It was an audio question which required me to write it as it was said in Klingon, not to translate the English phrase to Klingon. So when it was shown translated as 'Can we fix it?' I was confused because I only knew it without the ['oH].
I got it wrong because I didn't understand why ['oH] would be there (since I understood the sentence without it) and so I left it off so the correction would tell me what exactly was being said. So it wasn't anything Duolingo did wrong.
I haven't gotten to the pronouns lessons and learned GhaH yet, but when I do I can review this. :)
Thank you for the prompt response!