The plural markers are optional in Klingon, so a word without a plural marker, like targh, can mean either one thing or multiple of that thing, so "a targ" or "targs". Context usually makes it clear which is intended and the plural markers can be used for clarity. Thus we have to accept both "a targ" and "targs" as possible answers (and also "the targ" and "the targs"). You should not be marked wrong for any of them, but sometimes the system may show you one answer and sometimes it may show you another.
Thank you, Jeremy, for the explanation. I'm aware of the rules governing the plural. But it was new to me that duolingo sometimes chooses the plural as the correct answer and marks (I forgot to mention - and using my mobile phone I was a bit brief) the singular as a typo. Cheers, Steffen
It should not be marking the singular as a typo here. Did you include an article (like "a" or "the")? That's required with the singular. Is it possible there was some other typo? It's not impossible that it was a programming bug. Do you have a screen shot of it, by any chance?
It does not sound so rounded to me, but I suppose the rhotic postalveolar approximant of -rgh causes some rounding of the vowel.