UnityWebRequest return Unknow Error code 0

I write a very simple script to read the string from my website:

Code (CSharp):

1.  
2.     void Start()
3.     {
4.         StartCoroutine(SendRequest());
5.     }
6.  
7.     IEnumerator SendRequest()
8.     {
9.         UnityWebRequest uwr = UnityWebRequest.Get("https://www.gzztech.cn/");
10.         yield return uwr.SendWebRequest();
11.         if (uwr.isHttpError || uwr.isNetworkError)
12.         {
13.             Debug.Log(uwr.error + " " + uwr.responseCode);
14.         }
15.         else
16.         {
17.             Debug.Log("Get:Success");
18.             string a = uwr.downloadHandler.text;
19.             Debug.Log(a);
20.         }
21.     }
22.  

return error : "Unknown Error 0". the url just return a very simple string. I can open the page in any brower.
what's the wrong?Thank you!

Unity version : 2018.3.13f1

 

Verify you can connect to a site with http instead of https first.