Cancelling a free trial is done at least a day before the subscription start date with the steps here:
https://support.apple.com/en-us/118428?device-type=iphone
If you do not cancel at least 24 hours before the subscription start date, then you will be charged for the subscription and if that charge shows as Pending or unable to be processed, you won't see it in the article above to cancel because that subscription is till Pending. For a Pending transaction, you will need to let it proceed first, then you will be able to cancel to prevent future renewals. Are you seeing that charge as Pending or showing in Red in your Purchase History by following the steps under See your purchase history on your device here:
https://support.apple.com/en-us/118212?device-type=iphone