Get refund
How do I get a refund after being charged subscription fee for iTunes?
iPhone 6, iOS 12
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:
https://support.apple.com/HT204084
However Apple have been known to be sympathetic in cases of error, so it's possible that they may allow you a refund as a one-off concession: however please be aware that they are under no obligation to do so.
Go to https://www.apple.com/emea/support/itunes/contact.html or https://reportaproblem.apple.com/?s=6 as suggested in the page linked to above.