How do I cancel my Nebula subscription?

I suspect your Nebula subscription was not made by an in-app purchase, so it is not one that is being managed by Apple. You can verify if that is the case by using the link provided by FoxFifth to see if the subscription is there. If it is not there, then Apple cannot cancel it. Most users find they have a subscription after following a link to a post on Social Media or website advertisement that offered a Free Reading. In the fine print, they include the details the Free Reading involves a free trial that will become a subscription. That subscription is managed by Nebula and Apple is not involved at all. You will need to cancel the subscription in that case with Nebula.

There is an Apple article that explains how to cancel subscriptions. Click on the following link: If you want to cancel a subscription from Apple - Apple Support