The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/guidance/public-sector-procurement-from-1-january-2021?utm_source=e52d6bab-485d-476e-aafb-4fdff2b38d9d&utm_medium=email&utm_campaign=govuk-notifications&utm_content=daily