Remember that nobody on this forum works for Apple. We can't actually "do" anything about this other than provide you with information. If you download something by accident and start a trial then you need to ensure any subscriptions are canceled. You do not say why you found this to be "impossible". Read the information below about doing this. Sometimes people simply do not scroll down far enough to see the cancel option. If you now owe money you will have to pay it before being able to do additional downloads. If the item cannot be refunded then you are welcome to contact Apple but you may also just have to chalk this up to being a learning experience.
We all make such mistakes in life.
Confirm you have canceled any subscription automatic renewal (if you simply delete an app this does not cancel subscriptions):
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 in the EU from a third party, see: About purchases not billed through the App Store or the App Store's In-App Purchase system - Apple Support instead.
• "If you signed up 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."
• 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 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
