Messages won't help, it's the essential collection code the buyer should have brought for you to scan, that is the only way to mark an item as collected.
Send the buyer a message if they can kindly look through their emails, also Spam, their eBay messages, order details for that 6 digit code. If they can send that to you as this releases your payout for the item sooner. You can enter the numbers manually against where it says... Confirm Collection.
If not, then it's a minimum of 14 days for the release of that payout. As you are a new seller your initial payouts can be held for 21 - 30 days.
For any future collection items, never release the item without first receiving the code, a message to the buyer to explain this or even writing this in your listing may help with future sales, and quicker payouts.
@ma159582