To cancel a subscription/free trial that is managed by Apple, you do that here:
Cancel a subscription from Apple - Apple Support
Free trials must be cancelled at least a day before the trial ends to avoid the subscription charge as seen on the Approval Screen, requiring you to double click the side button, when the trial was started. If you did not cancel at least a day (24 hours) before the subscription start date, then that will be a Pending Charge when the transaction cannot be processed. You can view your Purchase History by following the steps under See your purchase history on your device in this Support Article. A transaction in Red or showing as Pending has not been paid.
View your purchase history for the App Store and other Apple media services - Apple Support
When did you cancel the free trial and when was the start date of the subscription? If you did not cancel in time and see the Pending charge in your Purchase History, then you will owe that amount and it is too late to cancel the free trial. If you cancelled more than 24 hours before the subscription starts and see the Pending charge in your Purchase History, then contact Apple directly, they should be able to see that cancellation date/time as well.
Contact Apple Support - Apple Support
If it is not in your Apple Account Purchase History and the description of the charge attempt you are seeing is not from Apple, then that was a transaction you made directly with the developer.