CI 3.0.7 don't see APPPATH. Why?

Hi!
My controller

PHP Code:

public function getorders()
    {
        $apiKey = '*****';
        $apiSecret = '***';
       
        require_once APPPATH.'libraries/Client.php'; 


Why is CI don't see libraries folder? 
Severity: Warning

Message: require_once(C:\OSPanel\domains\site3\application\libraries/Client.php): failed to open stream: No such file or directory