"Is it difficult to learn Chinese?"
The version given here seems a little awkward. You would probably be more likely to hear"汉语很难学吗?"
学 by itself in a context such as this is probably not going to work. It just isn't natural.
Came here to say that very thing. Also, 中文 should be a suitable substitute for 汉语 but Duolingo often marks it wrong, as they did here.
No native Chinese speaker would say it like that. The correct sentence is 学习汉语难不难？吗 variation hardly be used often