"Breakfast: bread and milk."
Duo is trying to reinforce the rule that French nouns most often have a modifier. There are exceptions, one of which is when a noun is used in a title.
Whether a noun is being used in a title depends on context and sometimes simply on what the author intends. Without context as this phrase is, it's hard to be completely sure.
If you left the article out because you were sure that this was an example of this or another kind of exception to the rule then you are right to feel disappointed that you lost a heart.
But if you left the article out because you forgot or didn't know that French nouns almost always require a modifier regardless of what the practice is in English, then Duo was correct to take your heart.
I think I should tell you that if you make a couple more such mistakes Duo's Owl will be very disappointed and start to cry and need to go into therapy.