In order to enable Google Maps you will need to a Google API Key follow this link and log in with your Google account: https://cloud.google.com/maps-platform/#get-started
Once you have your Google API key you will need to add it to opCommon.nmis to enable Maps in opCharts. You can find opCommon.nmis in /usr/local/omk/conf/opCommon.nmis, edit the file with vim (or another suitable text editor) and search for the opcharts_map_api_key property.
Open Open-AudIT Professional/Enterprise and select Admin -> Configuration -> All from the menu. Search for maps_api_key and place your API key in the value position.