The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/eu-settlement-scheme-employer-toolkit?utm_source=f459d4e2-e0b7-40d9-8319-dfb3f731d96a&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate