Exactly! I am unable to add additional addresses. I am not trying to change primary email address. I have attached screenshot of error.
Given the error, that email has already been used by another account.
As in your previous comment.
The purpose is to add all email addresses associated with other accounts to manage under 1.
So you can't use it, essentially what you are trying to do manage more than one account (multiple installations with their own my.avast.com account) isn't possible in your account.
Unless you have them all linked/connected (from their installation) to your current my.avast.com account and I'm not entirely sure this will work under the current my.avast.com function. Some time ago this was possible and was commonly used for family members, children, elderly parents, etc.