how do I cancel Disney plus. I can't find my iTunes account

You don't need to download iTunes on a Mac. The iTunes app for Mac computers was discontinued and replaced when macOS Catalina was released. It cannot be installed on Macs that are running Catalina or later. See the following Apple article for information about the replacement apps: What happened to iTunes? - Apple Support

You can cancel from an iPhone or from the App Store on a Mac or from iTunes on a Windows PC. 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

Or to cancel subscriptions in the Apple TV app, see instructions in: Manage subscriptions in the Apple TV app - Apple Support

Cancel,

Well, if you are paying for the subscription, there must be an account somewhere! Please try the steps in this guide:

If you want to cancel a subscription from Apple - Apple Support