The customer authorization (provided in the link you clicked) is out of date, and does not entitle you to use the Blended Threat service.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/1957651bdd7f1cf46ba1b95abb297158?link=https%3A%2F%2Fwww.gov.uk%2Fgovernment%2Fnews%2Fkickstart-scheme-opens-for-employer-applications