"The building is big but it is not pretty."
Translation:tIn qach 'ach 'IHbe'.
There are two words for "but" in Klingon: 'ach and 'a.
We only teach 'ach in this course but we accept 'a, in case a learner knows this word from elsewhere.
Single-letter typos that do not create a different word are usually silently accepted by Duo, so it thought that 'Q was just a single-letter typo for 'a rather than a big spelling error for 'ach.
Similarly, 'IcHbe' is just one letter off (an extra c) from 'IHbe'.
I'm a bit surprised that it let mach for qach slide, though, since mach is a real Klingon word.