That's not really a Duolingo issue, though, as much as it is just English being itself.
In most dialects of English, 'bread' doesn't take an article. It sounds funny to say, "I'm going to the store to buy a bread." Instead, you'd say, "...to buy bread." Or, if you wanted to specify an amount, you'd use "a loaf of bread/two loaves of bread". It doesn't actually matter that it takes an adjective, the rule for the article doesn't change.
Hence "a sweet bread" being marked wrong.
(And yes, you could argue that Duolingo could be less picky given it's a small thing and doesn't change the meaning, but that'd be a separate discussion.)
Well, ‘a bread’ does have a meaning in English, which is a type of bread. So ‘The girl is eating a sweet bread.’ means that the type of bread that the girl is eating is typically sweet (not that the specific slice that she is eating is sweet, although it would tend to be).
Whether the Hebrew can mean that, I don't know.
I disagree with those who disagree with you, for two reasons, and if you encounter that issue again I think you should flag it and say that your answer should have been accepted.
The phrase "a sweet bread" is perfectly acceptable in English. Another example would be "challah is a sweet bread." You should have no trouble finding examples of that phrase in print, especially with an internet search.
Those who base their disagreement on the phrase "a bread" are giving you a reasonable answer to a different question. But even with that phrase, their answer, while reasonable, is not comprehensive. There's nothing wrong with saying, for example, "She is eating a bread that I've never tried, and it looks delicious."
Is that what the Hebrew sentence was saying though? I don't think it was. If the Hebrew sentence was intending to contrast her eating sweet bread rather than another type of bread then you are right. But if the Hebrew sentence was just saying she is eating bread and it happens to describe the bread as sweet then you are wrong. I am pretty sure the Hebrew sentence was doing the latter meaning not the former so the English translation should not be "a sweet bread".
Learning a foreign language is a lot more than just learning a vocabulary list and then assembling the words as in English. Not only is the grammar different, but translations may be inexact, and a word that has multiple meanings in one language may not have the same set of meanings in the other language. We need lots of patience and practice.