The email address that you are trying to update to may already be your account’s ‘secondary’ email. If that is the case, you will need to delete the secondary email address before you can update your primary email address.
As a site admin, you can confirm that the email is your secondary email address by going to your admin page and clicking the Show button for the Secondary Emails row:
Instructions for deleting a secondary email address from the Rails console are in this post: https://meta.discourse.org/t/rake-task-for-merging-users/89086/33