I have an existing Wordpress site, albeit just a skeletal one with a single page. I decided to enable multisites on it and dutifully followed the simple directions for doing so. However, when I reached the "Create A Network of Wordpress Sites" dialog under "Addresses of Sites in your Network", only the "Sub-domains" option was enabled; the "Sub-folders" option was disabled with an explanation something like 'You can't use sub-folders in an existing site.' I found this peculiar because any site would have to exist before initiating the Multisite creation process, and so what made mine different? Any help would be appreciated.
From the Codex article Before You Create A Network:
You cannot choose Sub-directory Install (for a path-based network) if your existing WordPress installation has been set up for more than a month, due to issues with existing permalinks.
The easiest solution is to export your existing site (if it's got content you want to preserve), install a fresh WordPress Multisite installation, and then import your existing content into the new site.
Subfolder MultiSites are only allowed when you don't have any user-created content. That is because if you have pretty permalinks enabled, the subfolders could conflict with the slugs of existing Posts, Pages, or other content types. So if you chose, for example,
/coolstuff/ as your first subsite but you already had a Post with a slug of
/coolstuff/, WP would not know whether you wanted to show the subsite homepage or the parent site's Post.
If you just have a skeletal site with one Page, and you want a subfolder MultiSite, create a brand-new WP install and enable MultiSite. Then either copy and paste the Page, or you can export/import it.