I'm going to guess you didn't actually cancel the free trial correctly, which means you're still expected to pay for whatever the trial was for. For example, deleting the app, does NOT cancel it or relieve your obligation to pay.
This is how you cancel subscriptions for something you chose to sign up for --> If you want to cancel a subscription from Apple - Apple Support
The second issue is you MUST have a valid payment on your Apple Account to download apps. So in addition to being charged for whatever you signed up for, it's requiring a valid payment method for downloading any apps.
You should reach out to Apple about the subscription you bought, but didn't pay for. You can reach Apple Support by clicking on the Support button at the bottom of this page, or if you are in the US, you can call Apple Support at: 800-MY-APPLE