If a user has been created and an error that they are unavailable to validate their user account. Please check user manager to ensure the account does not show as activated.
If the account show as not activated, you will see option to remove the account, please remove the user from user manager and re-create them and a new activation email will be sent.
If the user receives the error message but the account shows as activated you will require a new account, which will require a new email address. You cannot use the same email address twice.