How can I cancel a pending payment in App Store?
If you cancelled the subscription soon after you subscribed, and you explain it was an accident no payment should be taken.

Bear in mind that if you had a free trial before the subscription, and you don't cancel 24 hours before the end of the free trial, you may still be charged.


  Some subscriptions might offer a discounted or free trial. If you don’t cancel more than 24 hours before your renewal date, your subscription might renew.


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Make sure you follow View, change, or cancel your subscriptions.

Additionally, you can go to Apple's official Report a Problem. This is specifically for subscriptions.