Alternate Universe Exchange!

Hi! I was trying to sign up for the AU Fest. This is my first time using the prompt form on ao3 so I am not really familiar with the tag nomination process. I assumed I would just have to use the prompt form to create new fandom/relationship/au tags that you would later approve of but the sign up form says These additional tag tags in your request are not allowed in this challenge when I try to. Am I missing something? Thanks for running the fest :)