Setting wp_enqueue_script correctly

Your add_action has incorrect parameters. Use this code

add_action( 'wp_enqueue_scripts', 'custom_scripts', 10 );

Also, you must prefix filename with /, so correct function code is

function custom_scripts() {
    wp_enqueue_script('custom_js', get_stylesheet_directory_uri() . '/custom_js.js', array('jquery'), '1.0.0', false );
}