call_user_func_array() expects parameter 1 to be a valid callback, function

You have to change this add_action('admin_menu','mafongroup_admin_menu'); to add_action('admin_menu',array( $this, 'mafongroup_admin_menu' ) );. Cause you’r here calling a class method. So you’ve to give reference to the class.

And also if you put public before declaring mafongroup_admin_menu() function, it would be more readable and would be helpful in future I think. See the updated class code of your’s below-

class mafongroup_admin {

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

    public function mafongroup_admin_menu(){
        if ( current_user_can( 'edit_theme_options' ) ){
            $page_title="MAFON GROUP";
            $menu_title="MAFON GROUP";
            $capability="administrator";
            $menu_slug="mafongroup_admin_settings";
            $function="mafongroup_admin_menu_function";

            $mafongroup_menu_page_creation_method    = 'add_menu_page';
            $mafongroup_submenu_page_creation_method = 'add_submenu_page';

            $theme_options = $mafongroup_menu_page_creation_method( $page_title, $menu_title, $capability, $menu_slug, $function,'dashicons-image-filter', '2.111111');
        }
    }
}

function mafongroup_admin_menu_function(){
    echo "<h1>TESTING</h1>";
}


new mafongroup_admin();

Hope it helps.