If that was the first time you had a trial with the app on a 3 day trial and you really canceled 20 minutes into the trial, it should not have charged you. That's presuming there was a trial. Not all apps have that. If you were unfairly charged, you need to explain that to Apple.
The standard thing with an app that has charged you is to pay the money you owe and then ask for a refund (note that refunds are not automatically guaranteed). Until you do that you will automatically be barred from additional purchases. Review the information in this support article by clicking on the link to learn how to cancel a subscription from Apple. ➔ If you want to cancel a subscription from Apple - Apple Support
If you subscribed to this directly with a third party, see: About purchases not billed through the App Store or the App Store’s In-App Purchase system - Apple Support instead. You need to work directly with the vendor, not Apple.
Notes:
• "If you signed up [which involves very intentional steps by you or a user of your device] for a free or discounted trial subscription and you don't want to renew it, cancel it at least 24 hours before the trial ends." Note that by using an app like this you are automatically agreeing (always read the fine print) to this happening, are responsible for reading if this will happen, and canceling the subscription before the trial ends.
• Deleting an app does not cancel a subscription.
• An existing subscription will normally continue until the end of its term and may then renew automatically. Stopping automatic renewal is effectively canceling it. "If you don’t see Cancel Subscription, then the subscription is already canceled and won't renew." The "Expires" date means the current subscription will just run until then. Note special conditions for some countries: About canceling subscriptions in Germany, Israel, South Africa, or Türkiye - Apple Support
• Read the article to see what to do about subscriptions that do not appear in your list. If you still can't find a subscription, contact Apple Support using the link at the end of the support article.
• Canceling a subscription does not automatically start a refund.
• If you had a free trial previously, you do not get a second one and a subscription will start immediately.
If you wish to pursue a refund, follow the directions in this support article about how to request a refund for apps or content that you bought from Apple ➔ Request a refund for apps or content that you bought from Apple - Apple Support
Read this support article for information about how long it takes to receive a refund (if one is granted), and how to check the status of a refund for apps or content that you bought from Apple --> Check the status of a refund for apps or content that you bought from Apple - Apple Support
