I can see that an old Avast subscription is registered to my old but still main email address. I’ve searched for a confirmation email from notification@emails.avast.com and there is NOTHING. Thus I can’t find my Order ID which is a prerequisite for cancelling a subscription that doesn’t show up when I log on to my Avast account (if it was displayed there I’m supposed to be able to cancel it right there, but no). This is a vicious loop that allows you to keep charging me for a subscription that isn’t used. Please provide me with my order ID.
I don’t know what you have actually tried, but this may be another possible route.
If as you say you can’t recall your order ID, try 00000000, this seems to be a means of proceeding when you can’t find the order ID. You would need to provide the related email address (which you say you know).
You could also check out this option:
Sales - Orders and Renewals FAQ - https://support.avast.com/en-ww/article/Alternative-unsubscribe/.
Scroll down to the “How do I cancel automatic renewal of my Avast subscription?” option.