If you have simple delivery on the listing you won't be able to cancel the original label. You could contact ebay to ask if they can see what the problem is with the label. Or cancel the transaction (via the dropdown menu arrow for the item on your orders list - a refund will be issued in the process) and relist to start again.
If you want to contact ebay you can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
If you decide to relist you can disable simple delivery during the listing process if you prefer. Select Custom Postage under the Delivery Details heading.