I wish to cancel my subscription, and get a refund

Strictly speaking all purchases are final (after all, you can't very well return them): please see this page for information on requesting a refund: Request a refund for apps or content that you bought from Apple - Apple Support - note that just deleting an app doesn't cancel the subscription. (To cancel a subscription please see How to cancel a subscription from Apple - Apple Support)

However Apple have been known to be sympathetic in cases of actual error, so it's possible that they may allow you a refund as a one-off concession: but please be aware that they are under no obligation to do so: go to https://reportaproblem.apple.com/?s=6

this link should have all the resources you need

Subscriptions and Billing - Official Apple Support