Multisite converted to Single Site can’t add new plugins or new themes

Check your wp-config.php file and remove multisite code.

remove code like

define('WP_ALLOW_MULTISITE', true);

and

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'yoursite.com');
define('PATH_CURRENT_SITE', "https://wordpress.stackexchange.com/");
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

Also, check your .htaccess file and remove those multisite.