This error message occurs where the email address already exists in ParentMail, usually because the user already exists, or where two parents/contacts are trying to share the same email address for communications (which is not possible).
If the user has previously registered, ask them to start the registration process again, except this time selecting the I already have a ParentMail PMX account option. If they can't remember their password, they will need to click the Forgotten your password link.
If they have never registered before, please search for the email address via Users > List. If the email address is listed against the wrong parent, remove the email address and add to the correct parent.