Function settings_fields() not recognized (Uncaught Error: Call to undefined function settings_fields())

Nevermind guys,

I found the problem by myself..

Original and wrong code:

$this->pages = [
            [
                'page_title' => 'WT Domain Check', 
                'menu_title' => 'WTDomCheck', 
                'capability' => 'manage_options', 
                'menu_slug'  => 'wtdomaincheck', 
                'callback'   => $this->LoadSettingTemplate(), 
                'icon_url'   => 'dashicons-store', 
                'position'   => 117
            ]
        ];

Corrected line:

'callback'   => array($this,'LoadSettingTemplate'),