"The girl is not cross" would be "Chan eil an chaileag crosta" or something like that. The definite article is specifically excluded by not being there in the Gaelic sentence.
ETA: I see from above that I didn't know this a month ago, and asked the same question. I'm learning!
You need to report it when this happens, checking "my answer should be accepted". It's by no means certain that one of the mods or contributors will see a comment here and pick up on it. If you use the report button they will see it and can decide if they agree.
Having said that, I'm not sure that "angry" would be accepted here, because we're taught further on in the course that "A girl is not angry" would be rendered in Gaelic as "Chan eil an fhearg air caileag" and I'm not sure if "caileag" should be lenited there so please don't judge me...
The suggested translation for this exercise -- "A girl is not cross" -- should have "crossed" instead of "cross" to be consistent with other translations of Scottish Gaelic "crosta" in Duolingo. (Or at least the exercise should accept "crossed" as a good translation, which it didn't, at least not in my case.)