Warning: An existing WordPress network was detected fixed

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

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', 'mysite.it');
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.

Reset your wp-config.php file back to its original state, remove any MULTISITE references.
Delete the .htaccess file or move it temporarily.
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
```
Log out and Log back into your site and initiate the Network Setup.
Recreate the .htaccess file with the copy pasted data.
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.