"Brown cars are not beautiful."
Translation:A barna autók nem szépek.
It is just incorrect, I guess. Hungarian uses the article even in general statements like these.
But if you were to say that there is no such thing as a brown car: "Brown cars do not exist.", then you don't need an article:
"Barna autók nem léteznek."
Or even better in the singular:
"Barna autó nem létezik."
Or, "I will not get into a(ny) brown car.":
"Barna autóba nem szállok be."
Probably because these statements are more abstract(?). They are not THE brown cars.
Many words in Hungarian have dozens of synonyms. Best to stick to the vocab they are teaching here. (Add one synonym the number of correct translations doubles, add two it triples - many sentences already have hundreds of correct translations without synonyms) That said, the decision to use "autó" rather than "kocsi" seems odd.