We are just customers, like yourself. If Amazon can't help you find a missing account, we surely can't. It's very easy to accidentally open a second account. If you recently changed the password or email to your Amazon account, try signing in with your old sign in credentials. Trying to change the credentials seems to be how most people accidentally open a second account.
Also, as suggested above, if you subscribed on a different Amazon country site, other than amazon.com (US site), you need to contact customer service from the specific Amazon site you used. For example, if you used amazon.uk, then customer service at Amazon.com can't find your subscription to help you cancel it. You would need to go to amazon.uk and contact customer service from there.
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I'm being extremely clever up here and there's no one standing around looking impressed. What's the point in having you all? - Dr Who
If you signed up for the CBS All Access Channel through the Amazon website (not the CBS All Access app) then it would be in your list on the Manage Your Prime Video Channels page. If it is not listed then you didn't sign up for the CBS All Access Channel, you signed up for normal CBS All Access either through the CBS website or the CBS All Access app.In those cases you need to look at the CBS Help pagefor info on how to cancel (differs depending on how you signed up).
Had this same problem. I only have one Amazon account, CBS All Access says I purchased through Amazon, and it did not show up under Prime Video subscriptions. In fact, I actually have the option to sign up for CBS All Access under Prime Video. However, I did some digging and did find out where it was in my Amazon account and was able to cancel it. First I had to go to Your Android Apps and Devices. On the left side of the page, you'll see it defaulted you to Your Apps. Change it to Your Subscriptions. Now you'll see it but its not called CBS All Access, for me it was called Commercial Free CBS Full Episodes and Live TV. Under actions on the right you now have the ability to turn off auto-renewal. I don't know why they hid it here instead of under Prime Video Subscriptions because it took me forever to find it so I hope this helps someone else.