If you didn’t have an old avast forum account before, then no problem. It is now associated to your avast installation, you have to link that to your my.avast.com account (allows you to monitor/manage multiple installations in the one my.avast.com account).

When you try to login in the avast forum, your input is validated against your my.avast.com credentials. The idea being you don’t have to have multiple different email and passwords for associated things.

The first time you do it the details would be input via the avastUI > Subscription/Registration - the details in the avastUI screen would probably differ greatly from when you did it before as the avastUI has probably changed a few times since then.