As I understand it, The Offline Map feature is not available in some versions of DJI GO (like DJI GO 4.1.18). If you're using a version of DJI GO that does not include the Offline Map feature, then install an older version of DJI GO (like DJI GO 4.1.15), cache the maps with the Offline Map feature, and then reinstall the newer version of DJI GO you were previously using. If you know you're using a version of DJI GO that includes this feature and it's not displaying in the menu shown in the above screenshot, then try restarting DJI GO.
I could be wrong about this.