I'm well aware of what an idiom is. However, answering expectations makes perfect sense. As I said it works even going by a dictionary definition.
You seem to want to exclude any definition that is not the first or second in the dictionary (I had a writing professor like that he always called it "Conversational English" and would dock points for anyone who used it) however that particular example works fine as the protagonist is literally having a conversation with someone else not writing a formal essay.
You can answer expectations http://www.dictionary.com/browse/answer definition 20
You keep citing "definition 20" on an unofficial dictionary website. That's not doing you any favours.
The dictionary doesn't have any sources or citations for that definition. It's highly likely this formation was used over 100 years ago, in which case it is completely irrelevant to a piece of English written in 2017. Indeed, someone on the last page asserted we should hard Google search for "answer your expectations" and - lo and behold - we could only find sources from non-native English websites (mainly French and other non-English European sites) or historical examples. It's an archaic form.
It's not a question of whether it makes sense or is "definitively" right. What matters is how human beings speak and what this game's dialogue is trying to capture (teenagers in conversation, typically). You're saying "'answer' being used like this literally existed at some point in history; therefore it's correct, no matter what". That's like if I was working on a game about modern American teenagers and started making them use "behither", "thusly", etc. It doesn't matter if the use of the phrase/word literally existed - writing dialogue is about place, tone and context. If it's not used today, and it doesn't fit the context, it's wrong for this work.
I hate to play the education card, but I studied linguistics (including historical linguistics) at university for four years as well as writing and English – your argument goes entirely against linguistic and narrative convention (as well as logic).
Is it wrong because it didn't say "you have entered" at the beginning?
Yep. Either add "have" or change "haven't" to "didn't".