Customize Issue

I had just set up my subdomain. But I’m not able to customize my website. The live preview is not showing. When I click on customize, the customize tab says loading……..
There’s no preview. The problem is with subdomain only. How to solve the customize error.
I also did troubleshoot but it doesn’t show any error.
This is not a theme specific issue. The issue exists with all kind of theme I install or I already installed. However, there is no plugin installed and a default theme is activated that comes with WordPress installation.

The page I need help with: [log in to see the link]