For a Click and Collect item?
Look through your eBay messages and order details for the item in your Purchase History. If it has not been sent there , contact eBay Customer Services they will provide this essential code for you. Be aware, you only have 7 days from being notified that it's in store, to collect your item.
This is the easiest and quickest way to contact eBay Customer Services from the link below, for a Call Back option. I do recommend contacting CS very first thing in the morning when the lines first open, as there's more chance of Dublin answering
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open between 8 a.m. to10 p.m. on weekdays.
9 a.m. to 6 p.m. on weekends
@toonstar1977