Nouns seldom go without an article in English, so "an egg" is probably a more correct translation.
I don't think so - unquantified egg is much more likely to translate into the plural in Hebrew - ביצים
"Egg" is (usually) countable. "How many eggs are you eating?" "I'm eating one egg." "I'm eating an egg".
However, "scrambled egg" is (usually) uncountable. "How much scrambled egg are you eating?" "I'm eating three plates of scrambled egg." "I'm eating two kilos of scrambled egg."
It's possible, but rare, to do it the other way round.
In my dialect of English, we use "scrambled eggs" and quantify.
"How do you like your eggs?" "Scrambled" "How many should I make for you?" "Two or three."
In Hebrew, unquantified egg is more likely to translate to ביצים.
I would agree that correcting my english grammer (a vs an) is not the goal here.
I think Duo considers the correct English grammar and spelling important because sometimes misspelling a word can change the meaning (ie never tell your mom you ran out of milf when you meant milk), thereby making the translation incorrect.