Why is "케이크는 맛있습니다" not accepted? I'm 95% sure there is no difference other than formality.
You are correct. This happens on nearly every exercise, so report it every time.
케으크 맛있어요 is not a typo
why is 케이크가 맛있다 wrong? It says 케이크가 맛있는다 is the correct form, is it some grammatical rule?