It has a very similar meaning, but there is a Klingon word for "be ugly", so it's actually possible to say that more directly. In as much as "not be pretty" and "be ugly" aren't really the same, only one is really the correct translation of this sentence, even though the other is very similar and can be used in essentially the same circumstances.
I just learned that ach means but in Irish and now 'ach is but in Klingon. This is going to be easy to remember :)
Also notice how tIn resembles tiny. I once read that Mark Okrand picked English words,modified them slightly and then gave them oposite meanings to make them more alien. So it's not unlikely that tIn is actually derived from tiny. Also mach sounds a bit like mega, but it's association with small is easy to remember because it starts with an m like mini and micro.
The "ch" is pronounced very differently in those two languages, but if it helps you remember, then that's great.
I don't know that the "opposite meaning" is all that common, but there are a few words like that with "tiny" being the prime example. I don't know if that one is intentional or not, but there are a lot of examples where we know for certain that he has a joke in the creation of the word (like qID meaning "joke").
I've added it as an alternative now.
In the future, you might wish to use the "report" function for this if you are reasonably sure that your sentence should be correct -- there should be an option for "my translation should be accepted":
That said, if you want to add a comment or justify the addition, then a comment in the sentence discussion may still be useful.
Missing hints for words that start with an apostrophe is a known problem -- Duolingo developers know about it but we don't know when this will be fixed.
Until then, you will indeed simply have to guess and then learn from the correction that the system gives you.
Alternatively, you could use a Klingon dictionary.
It's a good idea to have a copy of The Klingon Dictionary anyway, and if you choose to buy Klingon for the Galactic Traveler as well, then between those two books and the KLI new words list ( https://www.kli.org/about-klingon/new-klingon-words/ ), you should have access to all the Klingon words that there are.
All the words from this sentence are in TKD.
Actually, because of the bug and because this is one of the early lessons, the words 'IH and 'ach are specifically mentioned in the tips and notes for the lesson: https://www.duolingo.com/skill/kl/Dialogue-I/tips-and-notes .
Did you read them before starting the unit? It's in the first section, the one on "Adjectives". It's always a good idea to read the tips and notes before starting any new unit. If you have the "crowns" feature, click on the little lightbulb next to START after choosing a new unit; if you don't have the "crowns", then the tips and notes should be at the bottom of the unit's page after you've opened it but before choosing a lesson from the unit.
I assume you're using the website rather than a mobile app -- I do NOT recommend using a mobile app to learn Klingon from Duolingo since access to the tips and notes is not a given there.
Another alternative is to wait until the bug has been fixed.
This bug is one of the reasons the course is still in beta -- it's not finished yet and not ready for "prime time", but only for more adventurous learners.