You cannot without the code.
Send the buyer a message to please send you that code , as that releases your payout for the item otherwise it's held for 14 days. If sent, you can manually enter the numbers in the order details to the left of ... Confirm Collection.
For any future collection items I would advise after purchase and payment, to send the buyer a polite message reminding them to bring that 6 digit QR code , as you cannot release the item with it.
@hyslop870