The app I want to cancel and not pay for again is not listed in my subscription, but I received an email saying it would renew and I would be charged if I didn’t cancel.

Hi deidra200,

Welcome to the Apple Community where some of us do our best to help each other.

You don't say what kind of subscription ? 

Wondering if you trying to cancel from your Mac or iPhone ?

and

Whether the e-mail from Apple, (or could it be a scam) ?

Apple has several support articles that may help:

How to cancel a subscription from Apple:

(Scroll to the device you are using, or to the Mac info.)

https://support.apple.com/en-us/HT202039

If the subscription that you want to cancel is not listed:

https://support.apple.com/en-au/HT212052

Get help with subscriptions not billed through Apple:

https://support.apple.com/en-au/HT207865

Hope something here will help you cancel ... at least 24 hours before free trial expires.

Cheers

Thank you for your reply, deidra200,

If the subscription that you want to cancel is not listed:

https://support.apple.com/en-au/HT212052

Checking out Apple's support page will likely help you cancel.

Else you might want to click the "Get Support" link to Apple at the top right of this page.

Cheers :)