Is this a collection code, a QR code for an item they are collecting from you, or from a collection point?
If from you, this is how to finalise that transaction after they have come, collected and paid you.
It is not mandatory to use that code. Indeed, many sellers do not use the app to run their seller accounts, and that is needed here for that code.
You need to do this now, only on the Seller Hub
https://www.ebay.co.uk/sh/ovw
go to Orders >> Awaiting Postage / Print Postage label, and then click the box on the left of the item.
https://www.ebay.co.uk/sh/ord/?filter=status:AWAITING_SHIPMENT
From the Postage tab above, use the drop down menu there, if the option to mark as collected is not there, then select Mark as Paid and Mark as Dispatched.
Dispatched is eBay's wording for a collected item.
If they need the code to collect from their selected store, a Click & Collect item, they can either find the code with their delivery order, if not, they need to contact eBay Customer Services, who will provide them with this essential code.
