You don't have to have the subscription email as such, just the wrong one you entered and they can try and track that and any other information, name, etc.
Cancellation is somewhat different:
See - Cancelling automatic renewal of the Avast subscription - See
https://support.avast.com/en-en/article/35.
From that page select the "How can I cancel my Avast subscription?" option.
- You should also be able to open your existing Account right click on the AvastUI icon bottom right of windows > Registration that should end up taking you to the same area given in the above information. This is essentially the same Avast Account used to logon to the forums.