I don’t believe this will help as it applies to the ‘Avast Passwords for Windows’ product and not the user set password for protecting the Avast Antivirus product settings.
Unless I have got the wrong end of the stick - it is when the user sets a password to access the Avast Settings, as in my screenshot in my Reply #1
So this may require the download and use of the Avast Uninstall Utility.
Clean Reinstall - 1. - Download the avast! Uninstall Utility, avastclear.exe https://www.avast.com/uninstall-utility and save it to your HDD. Make a note of the steps required or print the page.
2. - Download the latest Avast off-line installation file for your Avast Program https://files.avast.com/iavs9x/avast_free_antivirus_setup_offline.exe and save to your HDD.
I can only assume that you set a password to prevent changes to avast being made in the User Interface (UI) ?
The Settings password is different and separate to your Avast Account, which you say know the password for.
If that is the case I believe you would have to completely Uninstall it from the Windows - Programs and Features Add or Remove a Programs, restart and then Reinstall Avast.
So you aren’t unprotected during this period - first download and save the off-line installation file so you don’t have to go on-line unprotected.
See this topic that has links for the latest Avast Offline installation files - https://forum.avast.com/index.php?topic=328426.0