Translation:Generally, dogs run faster than cats.
Why are classifiers used here? Previously it was not required to include classifiers next to general statements, e.g. "Chó va meo là dộng vật" = "Dogs and cats are animals", but instead were used where a specific object/animal etc was meant or where specifying a quantity of said object/animal, e.g. "Con chó của tôi ăn thịt" = "my dog eats meat", or "sở thú có hai con voi" = "the zoo has two elephants". This sentence appears to specify a specific animal, i.e. "generally the dog runs faster than the cat" but that's not what was intended, and so is apparently wrong.
The real problem seems to be in Duo's command of English, especially where use of articles, both definite and indefinite, is involved. Problems with the use of English articles occur throughout the course. Particularly notable is Duo's tendency to use "the" before abstract nouns.