"The soup is tasty."
Translation:Zupa jest smaczna.
"zupę" is the Accusative form, used for the direct object of the sentence (Jem zupę = I am eating soup). As "the soup" is the subject of the sentence here, it needs to be in Nominative, which is "zupa".
"zupa" is a feminine noun, so the adjective "tasty" also needs to be feminine, which is "smaczna".
You used a wrong grammatical form, so this is definitely a mistake.
The adjective needs to match the noun it describes. "zupa" is a feminine noun, so the right form of the adjective is "smaczna".
Your "smaczne" is either neuter singular or 'not masculine-personal plural' (non-virile).