If during installation of a plugin, Wordpress asks for your hostname or FTP details. Then follow these steps:
Login to your server and navigate to /var/www/html/wordpress/. Open wp-config.php and add this line after define(‘DB_COLLATE’)
define('FS_METHOD', 'direct');
If you get "Could not create directory" error. Give write permissions to your wordpress directory in recursive as
chmod -R go+w wordpress
NOTE. For security, revoke these permissions once you install a plugin as
chmod -R go-w wordpress