If you are getting this error
[ERROR] Command "site:install", is not a valid command name.
after installing drupal console, it it because you are running the command outside a drupal installation folder. The confusion is the fact that most of us expect a “site:install” to go out and retrieve the drupal files for us but the command is not doing that.
The site:install allows us to install drupal from the command line. It DOES NOT retrieve Drupal files for you.
It took me a while to figure this out.