Translation:Do we eat at 9 in the evening?
The question of 'do we eat' versus 'are we eating' may be related to the question of which expressions are used in both languages to talk about a repeated schedule versus a one time event. For English, I think it could be argued that either can work for both, though I might be more inclined to use 'do ...' for repeated events. Do we eat (/are we eating) at 9 pm (tonight / every night [during our trip])?
There's nothing explicit in the Chinese sentence to help us pin it down to a specific day, but I am by no means certain that's excluded. Has anyone tried 'Will we eat ...'? If that's acceptable, then so should 'Are we eating' be, because the present progressive is a common alternative way to speak of the future.
Currently, the "correct" answer indicates "Do we eat at 9 in the evening?", but rejected "Do we eat in the evening at 9?". I would suggest there is no plausible reason for the English translation to be this rigid, even though the former is more frequently used. I would suggest Duo add a category of correct but not preferred if the want to communicate such.
In American English we use tonight to refer to evening... what are you doing tonight? What is on TV tonight, where do we meet tonight? "this evening" is also used, but is rarer. Oh, as when we say "What did you do last night?" We mean BEFORE you went to bed and after to got home from work or school.