How to add admin.php to WP Admin Menu Link

Looks you are adding pages the wrong way.

Here how it should look inside the class:

class My_Add_Menu_Pages {

    public function __construct() {
        add_action( 'admin_menu', array( $this, 'my_custom_admin_pages' ) );
    }

    public function my_custom_admin_pages() {
        add_menu_page(
            'Social Networks Auto Poster',
            'SNAP|AutoPoster',
            'haveown_snap_accss',
            'nxssnap',
            array(
                $this,
                'showPage_accounts'
            )
        );

        add_submenu_page(
            'nxssnap',
            __( 'Calendar View', 'social-networks-auto-poster-facebook-twitter-g' ),
            __( 'Calendar View', 'social-networks-auto-poster-facebook-twitter-g' ),
            'manage_options',
            'nxs-function_name',
            array(
                $this,
                'showPage_about'
            )
        );
    }

    public function showPage_accounts() {
        echo '<h1>Parent Page</h1>';
    }

    public function showPage_about() {
        echo '<h1>Child Page</h1>';
    }
}

new My_Add_Menu_Pages;