Although the WordPress dashboard doesn’t offer any settings to enable it, enabling multisite support in WordPress is really easy. There are a lot of multisite plugins out there that make this easy and come with a lot of much-needed features. But if you just want basic multisite support, you can enable it by adding one line of code to your wp-config.php file.
Your wp-config.php file is in the root directory of your WordPress installation. Open it and place this line of code in it:
Add it to the top right after the opening <?php tag.
Note: Before you add this line of code to your config file, press ctrl+f and search for WP_ALLOW_MULTISITE in the file.
If you find that above line already exists, just change false to true.