The word "저" and the ending "~어" doesn't get along. You can write either "저는 긴 치마를 입고 있어요/있습니다" or "나는 긴 치마를 입고 있어."
Since "wear" is a state verb, it would be "I wear a long skirt", even if the original Korean sentence is "입고 있어". At least "I wear a long skirt" should be accepted. Reported on Nov. 7. 2017.
It seems odd gramatically to begin with 저는 but end with 있어. Shouldn't it be 있어요? Or should it be changed it to 나는 긴 치마를 입고 있어?
Long and large are different? I put "I am wearing a large skirt"