When subpages don’t work, the reason is usually some misconfiguration.
Looks like you are on Nginx, not Apache, therefore the .htaccess file is of no use? There is an example of an nginx config file here: Running Kirby 2.5.9 Starterkit (nginx, Ubuntu, DigitalOcean)
If it is an Apache server despite what it pretends to be, then try uncommenting the RewriteBase line in your .htaccess file and set it to / or /starterkit.