This is a known issue. There are temporary solutions posted here https://support.advancedcustomfields.com/forums/topic/google-maps-field-needs-setting-to-add-api-key/
If you are using ACF 5 Pro 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.
