If a user in Participant Registration during the checkout process receives a window that says, "You are not authorized to use this feature. Please contact your Site Administrator with information about what you would like to do."
The User has not set themselves up as an Administrator for said account. All that is needed is to go to said Household, edit said Submitter, and check the box that says "Household Administrator". Save. Then inform said user to log back in and complete checkout.
What Does "Not Authorized" Mean During Registration Checkout?
The 'Not Authorized' error appears when a user isn't flagged as a Household Administrator on their account. To fix it, edit the submitter on the household record, enable Household Administrator, and have the user log back in.