Ask them to check the postcode. Sometimes eBay run the numbers and letters together, and there's no gap.
Ask them to check that, that the gap is where it should be in your postcode, hopefully then they'll be able to print the label.
The seller, not you will need to contact CS if the matter is not resolved. Send them this link , it's the quickest way to contact Customer Services.
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
@tony2404