Unable to communicate with ArcGIS Online. This feature requires external access to ArcGIS Online and cannot be used in a disconnected environment

Error Message

The following notice is returned when trying to configure ArcGIS Online's Utility Services with ArcGIS Enterprise.

Notice:
Unable to communicate with ArcGIS Online. This feature requires external access to ArcGIS Online and cannot be used in a disconnected environment

Cause

The Portal for ArcGIS environment has a forward proxy configured, but the system properties in the Portal Administrator Directory are not configured as per the Configure your portal to use a forward proxy server documentation. This can also be confirmed by tracking developer's tool requests where:

https://proxy.domain.com/portal/sharing/proxy?https://www.arcgis.com/sharing/rest/portals/self?f=json

net::ERR_ABORTED

Solution or Workaround

The incorrect port was specified for the forward proxy in the Portal system properties. Also, an inappropriate WebContextURL was included. The below modifications can be made to fix this issue:

1. Log in to Portal Admin > System > Properties > Update Properties. 
2. Modify the WebcontextURL parameter to use the Portal web adaptor.
3. Ensure that the appropriate machine names are included in the nonProxyHosts parameter.
4. Ensure that the correct ports are specified in the httpProxyPort and httpsProxyPort parameters.
5. Update the properties and restart the Portal for ArcGIS service.