ArcGIS Web Adaptor: Failed to get machines from server site

Error Message

Attempts to configure the ArcGIS Enterprise portal with ArcGIS Web Adaptor (IIS) in an ArcGIS Enterprise deployment fail, and the following error message is returned on the Web Adaptor configuration page. This issue occurs even though the correct fully qualified domain name (FQDN) and primary site administrator credentials are used.

Error:   
Failed to get machines from server site.

Cause

The ArcGIS Enterprise deployment is in read-only mode. Read-only mode freezes the ArcGIS Enterprise deployment and restricts modifications and operations for the portal, federated ArcGIS Server sites, and ArcGIS Data Store. Refer to ArcGIS Enterprise on Windows and Linux: Functionality restrictions in read-only mode for more information.

Solution or Workaround

Place ArcGIS Enterprise out of read-only mode before configuring Portal for ArcGIS with ArcGIS Web Adaptor (IIS). Refer to ArcGIS Enterprise on Windows and Linux: Place ArcGIS Enterprise in read-only mode for instructions.