Why was I charged for a trial app even after canceling on last day?

When you agreed to the free trial, there is also a message on the same screen that says you must cancel the free trial at least 24 hours before the trial ends to avoid the subscription charge. This is before you double click the side button to authorize the free trial/subscription. Cancelling on the same day the trial ends means it is less than the 24 hours. It is considered a renewal when the free trial becomes a subscription.

Disputing the charge with your bank means your payment method is no longer valid to use as a payment method in the App Store. You will need to add a different valid payment method to use the App Store including downloading free apps, updates, and continue other subscriptions.

Not having a valid payment method has no effect on your current programs you have, so if you are having trouble accessing a program, then that is for a different reason and you will need to explain what happens when you try. It may help to visualize what you are seeing by providing a screenshot as long as it does not include any personal information.