Help installing demo site
So If you still have everything on your machine, the thing going wrong here is the composer install bit. It looks like you have installed composer locally inside the folder but not globally. You have a file called composer.phar inside you happylager.test folder.

The way you have composer at the moment (locally), you cannot do composer install in your terminal you have to do php composer.phar install from within you happylager.test.

This should sort it for you and you will get the vendor folder from it. Then, simply follow the other steps to installing the demo. I strongly recommend you install composer globally though, it'll be easier for you in the future :)