How to fix “Warning: An existing WordPress network was detected.”

While converting a WordPress single install to multisite, the following error notice was displayed at the Network Setup admin page.

Warning: An existing WordPress network was detected.

It took me quite some time to finally figure out what I was doing wrong.

Mind you, the instruction in Network Setup page clearly stated that the PHP constant definition below should be placed before /* That’s all, stop editing! Happy blogging. */ in wp-config.php file and I was placing the code the end of wp-config.php.

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'rainbowcreed.dev');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

Solution

Adding the PHP constant definition above /* That’s all, stop editing! Happy blogging. */ fixed the problem.

If the solution do not work for you, performing the ritual below certainly will.

1. Reset your wp-config.php file back to its original state, remove any MULTISITE references.
2. Delete the .htaccess file or move it temporarily.
3. Drop the following tables from your WordPress Database, use a tool like phpMyAdmin if you are not comfortable directly modifying table data.

   wp_blogs
   wp_blog_versions
   wp_registration_log
   wp_site
   wp_sitemeta
   wp_signups
   wp_sitecategories

4. Log out and Log back into your site and initiate the Network Setup.
5. Recreate the .htaccess file with the copy pasted data.
6. Modify the wp-config.php file, remember to paste those lines above the line where it says

   /* That's all, stop editing! Happy blogging. */

Source: Stack Overflow.