Laravel 5.5 fail when serve

5 project and after install Laravel and make php artisan serve it fails.

PHP Warning: require(/var/www/html/sidbrint/vendor/autoload.php): failed to open stream: No such file or directory in /var/www/html/sidbrint/artisan on line 18

PHP Fatal error: require(): Failed opening required '/var/www/html/sidbrint/vendor/autoload.php (include_path='.:/usr/share/php') in /var/www/html/sidbrint/artisan on line 18

I'm using an Ubuntu Server and I don't know hat to do.

via Chebli Mohamed