The seller cannot provide this code, they don't have it.
Check your emails, your order details and spam in case it's gone there. If not found contact eBay Customer Services they will provide this code for you. You have 7 days from being notified that it's in store to pirck up your item.
This is the easiest and quickest way to contact eBay Customer Services, for a Call Back option.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
Lines open 8 a.m. - 10 p.m. on weekdays
9 a.m. - 6 p.m. on weekends.
Automated agents will be available on chat outside of the above hours.
I recommend contacting CS first thing in the morning as there's more chance of Dublin answering.
@maryminx