The Google Maps API requires a key with start 22 june 2016.
http://googlegeodevelopers.blogspot.se/2016/06/building-for-scale-updates-to-google.html
Can’t find a solution how to add a Google maps API key.
This has been reported here multiple times. The developer is aware of the problem an is working on a solution. https://support.advancedcustomfields.com/forums/topic/google-maps-field-needs-setting-to-add-api-key/
There is a fix for this in the new version of ACF Pro. You can get it by logging into your account and downloading from there. It is 5.4.0-beta3.