Your script is running before ACF is initialized. More than likely you’re including your script too soon or running it too soon. When are your enqueuing your script? You should probably do this on the acf/input/admin_enqueue_scripts hook https://www.advancedcustomfields.com/resources/acf-input-admin_enqueue_scripts/
Something not mentioned, but an added precaution is to make your script dependent of “acf-input”
Also ensure that your script does not run until the document is ready.
