"Breakfast is delicious."
I used "아침이 맛있습니다" and it marked it correct. I think that "아침" can be translated to "morning" (as in "morning meal") or as "breakfast" (the "meal" being implied from context).
"아침이 맛있습니다": This sounds weird. You cannot skip the word "식사" here, else it sounds unnatural. Even if you say this when eating breakfast, it's unnatural. However you could say it in the past, like "아침 식사가 맛있었읍니다." When you are at table, it's more natural to say just "the food is delicious" i.e. 밥이 맛있습니다.