This message is provided our hosting support team. for customizer loading stuck.
There seems to be issue with this theme: generatepress_child which was causing issue, on deactivating that theme the page started loading.
Therefore I request you to kindly get this theme checked with the help of you developer as this is the theme that is causing the issue.
when we deactivate generatepress-child theme it will work customizer. but when we activate child theme again laoding stuck issue occuered. please help me for this problem.