The browser of the kibana-users needs to be able to reach out to the internet. if you open the chrome-debugger, you should see a call to "https://catalogue.maps.elastic.co/v2/manifest" in the network tab. if the call fails, you should see an error in the network log.
Also found a similar discuss post here: see if it helps you: Default map service error, cant create custom region map either
Cheers
Rashmi