Installed in wrong folder.
Now unzipped into in public_html/grav-admin
running . . ./grav-admin/index.php gets a different error.
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator at xxx to inform them of the time this error occurred, and the actions you performed just before this error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
Yes, I agree with @dan-james. Just copy the complete contents of grav-admin (including hidden files) into the root folder (public_html) and then visit the site in the browser. This should automatically load a page which will create you a user, which you then use to log in.
You do not run index.php, you run a server. For Grav 1.6 navigate to the folder on the command line and run php -S localhost:8000 system/router.php, for Grav 1.7 run php bin/grav server. .htaccess, which is a dotfile that you need to check exists in the root folder of Grav, contains instructions for Apache and Lightspeed.
Since Grav is flat-file you can run it in any folder, and you can tell your server what folder or domain to serve it from. It’s not uncommon to run it as localhost:8000/grav, domain.tld/grav, or directly on the domain. It doesn’t have to be in the root folder of the server though. https://github.com/getgrav/grav/blob/develop/webserver-configs/htaccess.txt can be renamed to .htaccess and moved up a level; this folder with server-configurations ships with all installations of Grav.
Since you installed on Bluehost with the Admin-plugin, navigate to yourdomain.extension/grav-admin and you should be asked to create an Administrator account.