ensure your server supports multisite if it does then add custom code to .htacess and wp-confi files

adding the code define( 'WP_ALLOW_MULTISITE', true );

.htacess /* Multisite */ define( 'MULTISITE', true ); define( 'SUBDOMAIN_INSTALL', false ); // Set to true if using subdomains define( 'DOMAIN_CURRENT_SITE', 'example.com' ); // Your main site domain define( 'PATH_CURRENT_SITE', '/' ); // The path where the network is installed define( 'SITE_ID_CURRENT_SITE', 1 ); define( 'BLOG_ID_CURRENT_SITE', 1 );