Hi,
this is a known issue, we are working on it and we apologize for the inconvenience.
If we got it right, you tried to cancel the subscription in My Avast account on the webpage
https://my.avast.com.
If that's the case then despite the error message, your subscription should be cancelled. You can check that by refreshing the page after you cancel it and you shouldn't see the the cancel auto-renewal option anymore.
Please let me know if that works for you.
If not feel free to send me a PM including your email you entered during the purchase process and I will check whether the subscription has been cancelled. If not I will cancel it for you of course.
Thank you.