You have buyer pays postage on your listings and if they paid they will be automatically refunded in 30 days.
If that listing had free postage - seller pays on it you can cancel it here:
https://www.ebay.co.uk/claims/simple-delivery/refund/seller
The item has to be sent with a tracked service for you to receive the refund.
To mark the item as dispatched open the original Simple Delivery label. Then change the tracking number via View Order Details > Postage > Track Package > dropdown arrow > Add Tracking on a desktop browser.