Make sure you’ve installed php and the necessary modules before trying to create a new Laravel project.
$ sudo apt-get install php-fpm php php-mbstring php-zip
The order of
php is important as putting them the other way around you’ll find you get
apache2 installed when you probably don’t want that.
Then you should be able to create your empty project using composer without any complaints.
$ cd /var/www $ composer create-project --prefer-dist laravel/laravel [project]