When trying to enable the WordPress multi-site you might get the following message in the setup:
“Because your install is not new, the sites in your WordPress network must use sub-domains. The main site in a sub-directory install will need to use a modified permalink structure, potentially breaking existing links.”
What this means is that there might be a conflict between existing pages/posts and new sites. So if you previously had a page with the slug “mobile” and after the setup you create a sub-site called “mobile” well…you see the problem.
The easiest way to avoid that restriction ( keeping in mind to avoid the slug conflict above ) is by adding the code below to an active plugin or theme.
add_filter( 'allow_subdirectory_install', create_function( '', 'return true;' ) );
After that, you can run the setup as for a new website, when you’re done don’t forget to remove the code.