The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.
https://www.gov.uk/government/publications/inheritance-tax-inheritance-tax-account-iht100?utm_source=4dd616e5-2118-4716-b08d-e237d07ba9b8&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate