Yes, i see and it was different from the screenshot and that’s why I asked
Ok, now regarding the problem, you have a bunch of errors on the customizer page, starting from 404, 406 errors and including mixed content loading warning, can you tell me what happened to this site? have you changed url? or anything like that?
such a problem might be the server related but please check is your plugins are not doing this, deactivate all third party plugins and try again your problem, most probably this I conflict with third-party plugins.
Once you deactivate all plugins please check your problem again, if the problem is gone then this means it was a plugin conflict. Now, start activating plugins one by one and same time repeat your problem until your problem appears again, now you know which plugin was in conflict