Customization is not working

I recently started working on building a new website and chose the Sinatra theme for its design and functionality. However, I encountered an issue right at the first step. When I open the customization window, no content is displayed, making it impossible for me to proceed with customizing the theme.

Could anyone guide me on how to resolve this? I’d appreciate any suggestions or troubleshooting tips to get the customization window to load properly. Thank you in advance for your help!

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