P.S., I have a sneaky suspicion your issue might have something to do with needing to follow the instructions on the Network Admin > Settings > Network Setup page that states, in my case, the following:
Network Setup
The original configuration steps are shown here for reference.
1. Add the following to your wp-config.php file in /Applications/MAMP/htdocs/wordpress/ above the line reading /* That’s all, stop editing! Happy blogging. */:
define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'localhost');
define('PATH_CURRENT_SITE', '/wordpress/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);
2. Add the following to your .htaccess file in /Applications/MAMP/htdocs/wordpress/, replacing other WordPress rules:
RewriteEngine On
RewriteBase /wordpress/
RewriteRule ^index\.php$ - [L]
# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
Unless, of course, you’ve already done this!
