I want my money back from the subscription I just cancelled

You may cancel a subscription to an app --> See or cancel your subscriptions - Apple Support

You can also request a refund for a recent renewal of an in-app subscription purchase 

If you have purchased an app there is no way of getting the refund. However, you may try -->  Request a refund for an App Store or  iTunes Store purchase

At times subscription refund may not be possible, however you may change or cancel a subscription—or check if a charge from Apple that you don't recognize is for a subscription—tap or click this button:

Depending on how long ago you initiated the subscription, you can go to apple and request a refund if purchased through the App Store.

https://www.apple.com/shop/help/returns_refund

Why do you think you would get your money back?

Cancelling ensures that you will not be billed for it again in the future. It does not get you a refund for what you've already paid.

Cancelling a subscription does not automatically give you a refund. You should get an email titled “Your purchases from Apple” in which there is a link to a website where you can request a refund.