nie widzą. You learned Nominative (zupa) and Accusative (zupę), now it's time for Genitive.
Genitive is needed by some verbs, by some prepositions, but one of its most important usages in in Negations. If you had a sentence that needed Accusative, if you negate it, you need Genitive instead.
Oni widzą zupę. Oni nie widzą zupy.
That is important: only Accusative changes into Genitive when negated. All other cases just stay the same.
My variant "They don't see a soup" was accepted but should not as "zupy" is plural.
It is not plural, it just looks like it is.
It is genitive singular, but many feminine nouns have genitive singular=nominative plural. You need genitive after "nie widzę", so plural it would be "nie widzę zup"
In case you did not know :
Widzę zupę. Nie widzę zupy.
Widzę zupy. Nie widzę zup.
No. Not in this case.
"zupy" is indeed Nominative plural, but this sentence doesn't take Nominative. "widzieć" (to see) takes Accusative, it's negated here, and negated Accusative = Genitive. So "zupy" is Genitive singular.
Genitive singular and Nominative plural of feminine nouns are very often identical.
I wrote "they don't see any soup" but this was marked wrong, apparently the correct answer according to the feedback was "they don't see a soup". As a native English speaker (with 11 years of post-secondary education) I can confidently say "They don't see a soup" isn't a sentence any English speaker would naturally say; it's bordering on ungrammatical. "They don't see THE soup" would be reasonable.
Well, I asked a British native, and it apparently sounds ok to her. Of course, that's uncommon, but seems acceptable - provided that it's not in the main answer. And the main answer has 'the soup'.
'a' was probably suggested to you because it was the closest accepted answer to 'any'. And 'any' will now be accepted as well.
When you listen to the audio, the inflection in the woman's voice sounds like she is asking a question so I assumed it was and put "Do they not see the soup?" Of course it was marked wrong because it apparently wasn't a question, no question mark but she sure sounds like she is asking a question.