Send the buyer a message to check their emails, eBay message and in Purchase History the order details for the item, that 6 digit code is there amongst one of those.
They can then send you the code for you to manually enter it alongside Confirm Collection.
If they cannot find it, the case should close on its own after a period of time, if it doesn't when a reasonabble time passes contact eBay Customer Services ask them to close it, they should , if they can see enough time has passed for the buyer to open any item not received case.
If you need to, the easiest and quickest way to contact Customer Services is through this link, for a Call Back .
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
On weekdays lines open between 8 a.m. and 10 p.m. - 6 p.m.
Weekends lines open between 9 a.m. to 6p.m.
@jach3776
