The translation is wrong - you can't have 'one soup' or 'a soup' in English. Soup is uncountable, like water. Presumably in German this isn't so?
I agree the English translation here "Make me a soup today" is awkward, but soup IS a countable noun. Just not in this sentence.
If you worked in a kitchen, it would work. My brother would come home and say "I had to make three soups today."
That's because it is shorthand for "three orders/types of soup." Soup is uncountable, but if unitized in cups, bowls, orders, etc. then it is countable.
You could also use this sentence if you were saying "Make me a soup today - onion, or tomato or chicken or whatever you choose", or "Make me a soup today - you have one hour!". As always with Duo, life would be easier with fuller context! But the owl doesn't give us any so we just have to guess!
Possibly because of the position you have put "today" in? In English it does not go between the verb & its object. It could go at the beginning, or as it is here.