The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/farm-payments-if-theres-no-brexit-deal?utm_source=c0907845-b1bb-4117-b306-7c2ef52ff5c7&utm_medium=email&utm_campaign=govuk-notifications&utm_content=immediate