When I try to access to specific site /wp-admin, I got a 404.0 error

I have defined a new domain in Network Admin / Settings / Multi-Domains

e.g. http://www.activevacationguide.com

Add a new site with site address http://www.activevacationguide.com
I leave /site as the site address,

However, when I try to access to this site, /site/wp-admin. I get a 404.0 error.

Please advise.