This error is more than likely cause by a conflict with some other javascript that running on the site.
Have you tried disabling other plugins to see if you can narrow down the problem? If that does not work then it could be something to do with the theme.
A javascript error in one script can cause other scripts not to load an run properly. It could be that another script needs to be corrected.
If you can’t work out what’s causing the error on your particular site you can open a support ticket https://support.advancedcustomfields.com/new-ticket/
