By default, MediaWiki's subpage feature is turned
off in the main
namespace, but can be used on talk pages and user pages. See
Help:Namespaces. In namespaces where the feature is switched off, any
slashes (/) within a
page
name are simply part of the page name and do
nothing special.
To enable it on the main namespace, add to LocalSettings.php
$wgNamespacesWithSubpages[NS_MAIN] = true;
Noted, thanks.