Why won't this submenu page show? - My First WordPress Plugin

Starting to build my first plugin and I'm completely puzzled on why the submenu page won't show? The main page shows up, not the subpage.

I've run over the syntax what feels like a million times and I just don't see the gap on what I have here. (Page markup is complete, not shown, I'm just trying to get the menu item to show up.)

/* Create Menu Item */
function core_settings_menu() {

    add_menu_page(
        'Core Settings  Code Manager',
        'Core Settings',
        'manage_options',
        'core-settings-page',
        'core_settings_page_contents',
        'dashicons-superhero',
        90
    );

    add_submenu_page(
        'core-settings-page',
        'Code Snippets',
        'manage-options',
        'core-settings-code-snippets',
        'core_settings_code_snippets_markup'
    );

}

 /* Add Menu To Site */
 add_action( 'admin_menu', 'core_settings_menu' );