I’m editing public/wp-includes/functions.php file, not the core one. It used to work.
Ths is your problem, never modify files in the wp-includes folder under any circumstances!.
When tutorials and articles refer to functions.php, they’re referring to the functions file in the current theme. This file does not always exist but you can create it.
I would recommend you create a child theme, to avoid your changes being lost when that theme is updated. You can also create a plugin by placing a PHP file in the plugins folder, and placing a command at the top with /** Plugin Name: haxpanel's plugin **/ at the top. This will appear in the plugins list in the Admin dashboard for activation.
Before you do any of this though, fix your WP install by undoing the changes you made to wp-includes. Again, never modify the files of WordPress itself. The only time you should ever modify files in wp-admin or wp-includes is when you’re helping develop new WordPress releases. For personal sites, client sites, commercial or enterprise development, it is unnecessary, and in a lot of cases dangerous to modify those folders. Do not touch!