PHP Warning:
  require(/opt/lampp/htdocs/www/laravelproject/vendor/autoload.php):
  failed to open stream: No such file or directory
  in /opt/lampp/htdocs/www/laravelproject/artisan on line 18
PHP Fatal error: require(): Failed opening required
  '/opt/lampp/htdocs/www/laravelproject/vendor/autoload.php'
  (include_path='.:/usr/share/php')
  in /opt/lampp/htdocs/www/laravelproject/artisan on line 18

sudo chmod -R 777 laravelproject