The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.
http://www.acumen.hmrc.gov.uk/_/app/emailRobot/index/click/secret/11d7ff04771785c6b1494a761c132f7b?link=https%3A%2F%2Fwww.gov.uk%2Fguidance%2Fwork-out-80-of-your-employees-wages-to-claim-through-the-coronavirus-job-retention-scheme