As a business you have to join via the NHS website. You give all the business details, and within 30 minutes they email a bespoke QR poster which you print off and display at the entrance.
Maybe the venue you visited hasn't registered yet.
So I assume when and if football restarts with a few spectators you are going to have to scan the venue QR along with your ticket? That might be interesting when it comes to some of the "not going to get it but definitely going to defy lockdown" crowd. Like I said on the main thread it would appear your main option when it comes to defying the rules is to wander round aimlessly and maskless with a placard.