Adding subfolders to a subfolder on a group

I have a question for anyone and everyone on Fimfiction who knows how to do this.

CAN ANYPONY PLEASE FOR THE LOVE OF CELESTIA TELL ME HOW TO CREATE A SUB FOLDER WITHIN A FOLDER ON A GROUP?!?!?!?!?!

I was just recently made an admin of this group I am in called "Help get SUNSET SHIMMER on MLP FRIENDSHIP IS MAGIC" and I have been trying to create subfolders within the three folders I already have (Everyone, Teen, and Mature: to which I want to add tags for all of the story categories), but every time I try it keeps telling me I am doing something wrong and I do not even have the faintest Ideas as what I am doing wrong.

Can anypony please help me out or give me advice? I don't know what to do.