"Yes, I am busy on the weekends."
Hmm… possibly the non-Japanese comma…? That'd be a bit weird if so though… Maybe they didn't expect anyone to switch to standard commas mid-sentence?
Or if it was a type-what-you-hear lesson, I've heard they must be answered in exactly one way, so Julia might be right about it possibly wanting different kana (unless that's been changed; I don't actually remember how old those comments about "one answer only" were).
In Japan they will probably use Kanji. But duo uses kana to make it easier to learn and recognize the sounds. And sometimes uses Kanji to slowly introduce us to its usage. Could be better? Definitely, but i don't thing it is that bad. I recommend using a notebook while studying