You can also, if you do not want to post screenshots, tap the ? button at the top of any app and send in a quiz report - that includes visuals of your quiz for me to look into.
From the screenshot it seems you answered 「ぬ」while the quizz expected「nu」 (eg. you were asked on the pronunciation of the kana, not to write it down). Could it be that?
There's only two real types of kana questions: show the kana and choose/type the "definition" (the romaji spelling), or vice versa.
So, in this question, it was showing you ぬ in the question box. While it may seem obvious that are capable to typing out nu than converting it to ぬ, then you know the answer, but the system is asking for the romaji form (so that's what you need to write).
