Hello,
Because your server blocks request https://prnt.sc/117e86k on this installation.
Your hosting provider should answer why checking the server error logs. Very ofter it’s related to SubstituteMaxLineLength
Read this topic https://www.8theme.com/topic/internal-server-error-in-customize-mode/#post-272610
Tried to add the code using file editor but it caused the error on your installation. So, please, contact your hosting provider to resolve the issue with the settings because we can do nothing from the theme side when it’s related to server settings for this WP installation.
Regards