Make sure that you have the most current versions of WP and ACF installed. Make sure that you have cleared your browser cache. If the condition persists you might have a corrupt JS file, try reinstalling ACF. Also make sure that it’s not a plugin conflict by deactivating everything else. You might also have a JS conflict in the theme. Be sure to clear up any JS errors and warnings that happen before this error.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users