cannot connect to destination host with AWS Polly

Question

I am testing AWS Polly. I got an error saying 'cannot connect to destination host'. This happened right here.

UnityWebRequest www = UnityWebRequestMultimedia.GetAudioClip($"{Application.persistentDataPath}/audio.mp3", AudioType.MPEG);
    yield return www.SendWebRequest();

    if (www.isNetworkError || www.isHttpError)
    {
        Debug.Log(www.error);  <============
    }
    else
    {
        ......
    }

I signed up AWS and got some access keys as root user for Polly. Do I need to do something more to access AWS services? Thanks in advance for any help.