You cancelled too late. When you started the free trial there was a message on the Authorization Screen that required you to double click that said to avoid the subscription charge, you must cancel a day before the start date of the subscription.
Cancelling the subscription does not make the charge go away, it just prevents the next charge from occurring. You currently owe that charge and it is not going to go away. Eventually your account will be disabled and you will not be allowed to use the App Store for updating apps or even downloading free apps until it has been resolved.
You cannot request a refund on a Pending charge, so you would have to let the charge go through before you were able to request a refund. In most cases all transactions in the App Store are final, but at times Apple may provide a compassionate refund. It should never be expected though.