Why is Cô ấy muốn quả cam a correct translations, but Cô ấy muốn cái cam isn't, given that orange is an object. In the basic grammar tips and notes, "cái" was indicated to accompany almost every object.
Yes, "cái" can accompany almost every object, but not 100%. It also shows that for fruit, you must use classifier "trái" or "quả".
Thank you for the clarification!