How can I cancel this subscription?

In order to get a refund, you need to open your purchase history so you can submit a refund request to Apple.

Please follow the steps below:

1. Click this link: https://reportaproblem.apple.com

2. Sign in to your Apple ID that you use to purchase the subscription.

3. On the Report a Problem website, under the Purchases tab, click Report a Problem to the subscription of the app you want refunded.

4. Choose a reason for the problem, describe your issue, then tap or click Submit.

Based on the information that you provide, Apple might provide a refund directly, refer you to the developer, require additional information, or might deny the refund request based on refund eligibility.

See here: View, change, or cancel your subscriptions - Apple Support