You do need that code for eBay to release your payout , otherwise it can be held for 21 days.
Send your buyer a message to send you this 6 digit code, they'll find it in their eBay email messages or with the order details for the item in Purchase History. Then you can manually enter the numbers to the left of .... Confirm Collection in the Order details.
As a new seller, have you fully registered for eBay's Managed Payments? See here:
You need to provide eBay with details of a bank account and have this account verified. The usual method here, is for eBay to send 2 micro deposits into that nominated bank account for you to receive, and confirm to eBay.
Once this is completed, you will be able to list items for sale, and receive payouts from those sales.
This will show you how to register for eBay's Managed Payments.
https://www.ebay.co.uk/help/help/selling/getting-paid/introducing-managed-payments-ebay?id=4795
If there is an issue with the registration contact eBay Customer Services, who have full access to your account, and can see where the issue lies and explain the steps you now need to take.
@grjo550