Route ['/', 'index.php'] not defined.

I recently started with the PHP for beginners and was going totally according the series but after completing the episode 36 my code output on the webpage started showing "Not Found The requested resource / was not found on this server." which was called using $router->get('/', 'index.php'); This has made me mad for several days and I'm not able to figure out how can I fix the issue. I've tried copying jeffery's code from github and ran it over my device but it didn't work. I could really use some help.