Jesus tells them for the first time that he will die soon (verse 21). He will tell them the same thing many times after this first time. It is after this first time that Peter takes Jesus aside. (See: [[rc://en/ta/man/translate/writing-background]])
This is an idiom that means "may this never happen." Alternate translation: "No" or "Never" or "May God forbid this" (See: [[rc://en/ta/man/translate/figs-idiom]])