Admin only out of sync with admin rights
Issue
In some rare circumstances you can end up in a state where Trello doesn't correctly communicate to amazing fields about the status of a user's admin rights on a board. When this happens the user will not be able to edit the field settings if they are set to only allow board admins to open Amazing Fields settings.
Resolution
First, verify that you are listed as an admin user on the board. Click on the "Share" button on the board shown above. Then find your user name in the list of "Board members" and verify that you are listed as an Admin. If you are simply a "Member", then have the board owner change you to an Admin.
If you are listed as an admin, then the data being reported to Amazing Fields is out of sync with the Trello board. The easiest way to resolve this is to have someone remove you from the board and then reshare it with you ad as admin. They can do this using the same "Share" button above. Once this is completed you should be able to access the board as normal.