Add Admin to a Membership

Simple WP membership won’t allow me to add administration as a member and thus I cannot view or edit any pages in my builder when they are protected. Please can someone tell me how I can get around this.

The message:
This email address ([email protected]) belongs to an admin user. This email cannot be used to register a new account on this site for security reasons. Contact site admin.

For testing purpose, you can create another user account that is completely separate from the admin user account of this site.

Many thanks!
Adi

The page I need help with: [log in to see the link]