Thank you for being explicit for those who may be coming to Klingon from another language. I generally do it on my phone, and the only possibilities given for translating this sentence were "emissaries" and "emissary's," so I quite honestly did not know which one the program wanted. It turned out to be "emissaries."
Was this a tapping exercise with a word bank?
I think it's a general Duolingo problem with tapping exercises and words ending in an apostrophe in English: the apostrophe is not part of the tile, nor is there a separate tile with just an apostrophe.
Try typing just The - ambassador - is - the - emissaries - boss and see whether that's accepted.
Yes, I'm sorry for being confusing, but I tend to think of those as tiles. "Emissaries" was definitely accepted, I figured out the second time. I was just hoping you could change it so it had the appropriate apostrophe or could add an apostrophe tile, but as with the apostrophe on the front of the word, this may just be an artifact of the software. I was trying to think of other languages with important apostrophes, but I guess they never come at the end of the word.
I've seen people complaining about missing genitive plural apostrophes in several courses (e.g. Greek and German), so I doubt there's anything we as course contributors can do. (Short of never having English sentences with word-final apostrophes.)
You can report it as a Duolingo bug if you want (see https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug- ), but I'm sure they must have it on their list already, just not prioritised high enough for them to have got around to fixing it yet.
(BTW, one language I can think of that has apostrophes at the end of words is Greek: unlike, say, French, where words are written together when a word-final vowel is elided as in l'oiseau, in Greek, the words stay written separately, e.g. σ' αυτόν, απ' το βιβλίο.)