Advanced iframe debug console - l: local messages, r: remote messages
User agent: CCBot/2.0 (https://commoncrawl.org/faq/)

Headers of http://www.gmatjakarta.com:
HTTP/1.1 302 Found
Cache-Control: private
Content-Length: 123
Content-Type: text/html; charset=utf-8
Location: /Login
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
Set-Cookie: __AntiXsrfToken=d9709e94ea654bee97e655e7f28ec89f; path=/; HttpOnly
X-Powered-By: ASP.NET
X-Powered-By-Plesk: PleskWin
Date: Sun, 26 Sep 2021 06:27:13 GMT

HTTP/1.1 200 OK
Cache-Control: private
Content-Length: 6315
Content-Type: text/html; charset=utf-8
Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
Set-Cookie: __AntiXsrfToken=ef97a238263949a5afed6d7c1c9d7f11; path=/; HttpOnly
Set-Cookie: dis=; path=/
Set-Cookie: dit=; path=/
Set-Cookie: ausgang=; path=/
Set-Cookie: endtime_one=; path=/
Set-Cookie: endtime_two=; path=/
Set-Cookie: endtime_three=; path=/
Set-Cookie: endtime_four=; path=/
Set-Cookie: endtime_five=; path=/
Set-Cookie: counter=0; path=/
X-Powered-By: ASP.NET
X-Powered-By-Plesk: PleskWin
Date: Sun, 26 Sep 2021 06:27:16 GMT

The url you try to include is HTTP and your page is HTTPS. This is not supported by most modern browsers. See this blog for details. The url is redirected to http://www.gmatjakarta.com/login (http status: 200)!

Http iframes are not shown in https pages in many major browsers. Please read this post for details.