The post code is incorrect. Contact the buyer and check with them. You may have to cancel the transaction. Go to the dropdown arrow alongside the item on your sold page > Cancel order > Something wrong with buyer's address. . If they've paid the refund will be issued in the process.
You can relist the item and if the buyer can update their details on ebay they can buy and pay again with the correct address. If not or if you have any other concerns put their ID on your blocked bidder list:
https://www.ebay.co.uk/bmgt/BuyerBlock
@shlo_9690