Don't use that. They switched from npm to yarn at some point and forgot to update the docs. Start over and follow the github README. The are still some inconsistencies there, I think some folder names are messed up. Plus you need a Postgres database - do you have that? The installation script requires admin privileges on Postgres, at which point I gave up on it with a loud "WTF". But it might work if you have an new empty Postgres install.

As I said, major PITA.

And no, it's not PHP.