You cannot mark it as collected the only way is to enter that 6 digit QR collection code.
Send the buyer a message would they kindly send the code to you, it's found either in their email, eBay message or order details, and you can enter the numbers manually where it says Confirm Collection.
Never release collection items without this code, a message to a buyer before they arrive reminding them to bring it or the item cannot be released.
With no code to prove collection your payout will be held for a minimum of 14 days, possibly longer as you are a new seller.
@jameel_lalani