Another option: This sweater has one sleeve longer than the other
This sounds much more natural
the English is more likely to be "...one sleeve longer..." rather than "...a sleeve longer..."
Another acceptable answer should have the word longer before sleeve rather than after it. This sweater has a longer sleeve than the other. Reported June 23 2020
No. That means it has a longer sleeve than the other sweater. But we need to say it has one sleeve longer than the other sleeve. So Duo's translation is more clear.
I do agree with Penguin and Anne that "one sleeve" is better than "a sleeve" .
Agree that one sleeve should be accepted. It sounds more natural.
UK English one sleeve lo ger than the other
Now that we can see that "one" is accepted for "a" in this exercise, how about changing the article in the word bank, Duo? Is it because "a" is used in other (non-English) languages more commonly?