Hi, thanks for your reply!
Basically, I use here except I need to write about my private information.
By the way,
> All you need to do is go to ～～ "Connect to WordPress.com"
Unfortunately, it is NO. Actually, when I logged in my root site at the first time after the renewal, I found the button "Connect to WordPress.com". I was wondering why I need connecting to WordPress.com about my root site again, I clicked the button, but I got the error message above I wrote and the button that says "Connect to WordPress.com" still stayed there.
After reading your reply, I tried it again and got the same result. And I tried to connect WordPress.com Stats by the old ID you gave from the web browser address bar, then I had the message below.
You are not a member of this site.
If you have just installed the WordPress.com Stats plugin, this error probably appeared because the API key entered in the plugin belongs to a different WordPress.com user account.
The owner of the API key must visit this site's stats page to grant access.
You are logged into WordPress.com as o6asan. To switch accounts, log out.
My root site is the parent of a WordPress Network. The old test site does not exist on the Network anymore, so, a person reaches to my root site if the person accesses the old test site because of WordPress Network rewrite rule. This behaviors might be the reason I cannot resister my root site as a new site on WordPress.com.
If I recreate a new site which has the same URI of the old test site, does it make an effect or give more troubles? I have not tried it yet because I'm not sure about it.
Or can they fix their databases for me if I ask it to WordPress.com?
I need assistance. Thanks.