No cancel button appears to cancel my subscription to Calm

If the option to cancel a subscription is missing, see the following excerpt from Apple Support article If you want to cancel a subscription from Apple - Apple Support

You might need to scroll down to find the Cancel Subscription button. If there is no Cancel button or you see an expiration message in red text, the subscription is already canceled.

Also, a subscription that is already cancelled will typically have wording in fine print similar to "Expires on [date]" rather than "Renews on [date].”

The lack of a cancel button normally indicates it has already been cancelled as explained in the very support document you posted from.

If there is no Cancel button or you see an expiration message in red text, the subscription is already canceled.