If a contact or staff user receives this message when they click the registration link, this means that they have already registered.
Search for, then select the required user. Their profile is displayed on the right-hand side. If they show as Connected, they have already registered. Please advise the user to login, instead of trying to register again.
If a contact user receives this message, ensure their contact details are stored on their profile rather than on the child's profile.
If any other user (e.g. Governor, Volunteer) experiences this error, it means that they have not been assigned the Staff user type. Ensure that this has been added as described in this article, then resend the registration link.