If you have had WordPress installed for longer than a month and are attempting to activate the network, you will be told to use Sub-domain sites. This is in order to ensure you don’t have conflicts between pages (i.e. example.com/pagename ) and sites (i.e. example.com/sitename ). If you are confident you will not have this issue, then you can change this after you finish the initial setup.
In your wp-config.php file, you’ll want to change the define call for SUBDOMAIN_INSTALL:
define( 'SUBDOMAIN_INSTALL', true );
define( 'SUBDOMAIN_INSTALL', false );