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Firstly, Czech and as far as I've seen here Russian as well, have different feeling for the daytimes in comparison to English: вечер starts roughly at 6pm and takes till midnight. ночь is from midnight till lets say 4 am. Therefore the question "What did you do last night" in Russian is kind of silly, because unless I partied till 2 or 4am the answer would always be "Well, I splept, what else did you expect?"
While if you want to hear what he had done "last night" in the meaning "after 8pm before going to bed", you have to use the word вечер.
And I have no idea why it wasn't accepted in the other sentence, but it's probably because the way how DuoLingo works, every single sentence is inputed manually by different people and no one revised that sentence yet.
вечер stands for just time when you would expect dinner till 10 pm. ночь on the other hand stands for from whole night as evening and night do.
Easily you can ask yourself, what would you could ask about just as an evening even or whole night, usually themes of a question tend to be closer to one of those.
вечер starts at about 5 pm and lasts till you go to bed. Then comes ночь. It lasts till you wake up or till the dawn breaks, but in the far north (e.g in St. Petersburg) in early summer we have the so-called "white nights" when it never gets dark. In the astronomical sense, ночь is the time between sunset and sunrise of the following day. Утро starts at dawn and lasts till about 11 am (not till noon like the morning in English). День lasts from sunrise till sunset so it includes late morning and early evening. Russian doesn't have the concept of the afternoon, so in Russian we say "вторая половина дня". When you speak to a Russian audience in the afternoon, it would be wrong to translate "this morning we discussed ..." as "сегодня утром мы обсудили...". The correct way to render this thought in Russian is "В первой половине дня / До перерыва на обед мы обсудили..."
They are not interchangeable. "Ночь" is night, "вечер" is evening. The problem is that English speakers tend to use "night" instead of "evening" in contexts where Russian speakers would say "вечер". Essentially "ночь" is the time when you are sleeping or at least supposed to be. "Вечер" is the time before that, when it's late but too early to go to bed yet. It's the time when people go to restaurants, cinema, concerts or whatever.