I’m being charged for a Nebula subscription and I don’t have the app

You don't need the app, you were directed to their website for a free reading, most likely from a link on a Social Media site or a website advertisement. The free reading also included a trial version of their service that you are now being charged for. Apple is not charging you and has nothing to do with this subscription. Apple can only manage purchases/subscriptions when they are made through an in-app purchase, any subscription made on a website must be cancelled there.

Here are the four usual cases, and what to do about each: Billed, but you didn’t buy? - Apple Community