You cannot now mark this item as collected as you rightly say, that QR code was not available.
The notice to post will soon stop, but you will now have to wait at least 14 days for your payout.
If this happens again, best to cancel the order with the buyer's agreement, and relist with Collection only option. Best to do this, as you do leave yourself open to a case for item not received, and no proof that it was.
@vinyl_junkie21