You don't say "a tomato soup." You would say either "tomato soup" or "some tomato soup." This translation is a literal translation, not a true English translation. This is incorrect English.
Google "countable" and "uncountable" nouns. You don't use the indefinite articles with an uncountable noun such as soup, milk, cheese, etc. You do in some other languages, but not in English. You may hear this used, but usually not from a native speaker.
You would say a bowl of soup or a cup of soup, but not I would like a soup.
Chiming in a little late here, but I think I have a situation where you might say "I'll take a tomato soup." If someone were to be helping you get your cans of soup. Perhaps they are too high up on the shelf, and you might say, "I'll take a tomato soup." or possibly "I'll take a tomato soup and a chicken noodle." Maybe in a situation like that, it being one of many cans.
Good example. Personally I might also order "a tomato soup" at a restaurant. I'm a native English speaker and didn't think the sentence sounded strange.
That may be true, but a native English speaker would not think of that. But I guess that's why we have Duolingo to teach us.
I have reported this because, although in both languages there may be times when soup could, possibly, be a single item, in English, we simply do not say we want a tomato soup. Anyone who argues otherwise is undoubtedly correct in some wildly rare occasions, but the vast majority of instances it simply doesn't occur. Soup falls into the category of those nouns whose constituent matter is grammatically nondiscrete. I challenge you to show me a soup. You won't be able to. You will show me a can of soup, or a spoonful of soup, or a drop, or a bowl or what have you, but you can't show me a soup any more than you can show me a water. Because the vast majority of English sentences would not include the article, Duolingo should accept those sentences. There are times when these sentences are bizarre, in which case we just shrug and carry on, but this time it is simply wrong.