Customize not working

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. My main website [ redundant link removed ] has everything fine. How to solve the customize error.
I also did troubleshoot but it doesn’t show any error.

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