@anon60995106 if you are invited to an Organization as a Guest you have the ability to interact with Custom Fields but you can’t create them. It’s one of the few things a Guest can’t do.
Please visit this post to learn more about Guest Permissions: What if a friend invited you to an Asana-themed amusement park as a Guest? 🎟 This tip highlights the rides you aren’t tall enough to ride! (Guest Restrictions)