Attempts to validate an ArcGIS Data Store relational data store fail, and return the following error messages in the ArcGIS Server Administrator Directory logs:
Error: The connection property set was missing a required property or the property value was unrecognized. Instance not available on server
Error: Failed to create the service.: ERROR: unable to connect to the database: <MachineName>. Reason: Instance not available on server
Missing or inaccurate information in the ArcGIS Data Store configuration file causes the validation to fail as ArcGIS Enterprise is directed to a different or non-existent data store in the deployment.