"My dog sells hats."
"わたしのいぬはぼおしをうります"、what if I can't write kanji on my computer. I think that hiragana should be accepted.
They often accept hiragana as well (though not always). You wrote ぼおし instead of ぼうし, it might have been that. If you wrote it correctly and it still wasn't accepted just report it.
It didn't just give you tiles to choose from? I've yet to see a lesson where duo makes you type in Japanese bcuz what if people don't have the right keyboard? But if you can type the hiragana, you should be able to type the kanji
Web Version you get exercises where you have to type in Japanese. The app can't do it, because ???
I lived in Japan for 3 months and used this sentence every day! Great teaching, Duolingo! Keep it up! 【皮肉危険！】