I tried "you don't eat fish sandwich" and lost a heart. Is it really wrong not to put that "a" there in this case?
It's ungrammatical without the article.
It doesn't make sense, 'You do not eat a fish sandwich.', not something I would say in English, more likely to say 'You are not eating your fish sandwich' (of course many sentences her make no sense)
German doesn't have a separate present progressive tense, so 'ich esse' can be 'i eat' or 'i am eating' depending on the context. So 'you are not eating a fish sandwich' should also be correct in this case.