ERROR 999999 Something unexpected caused the tool to fail

Error Message

When working with utility networks in ArcGIS Pro, attempting to run the Add Rule or Import Rule geoprocessing tool may return the following error message:

Error:   
ERROR 999999: Something unexpected caused the tool to fail. Contact Esri Technical Support (http://esriurl.com/support) to Report a Bug, and refer to the error help for potential solutions or workarounds.
Unexpected operation [Error updating row from table ..UN_6_Props]
Unexpected operation [Error updating row from table ..UN_6_Props]
Failed to execute (AddRule)

Cause

This error is displayed when the existing enterprise geodatabase connection in the ArcGIS Pro project has the versioning type set to Traditional instead of Branch in the Geodatabase Connection Properties window.

Solution or Workaround

Set the enterprise geodatabase connection to the Branch versioning type in Geodatabase Connection Properties.

1. In the Catalog pane in ArcGIS Pro, right-click the enterprise geodatabase connection, and click Geodatabase Connection Properties.

2. In the Geodatabase Connection Properties window, under Versioning Type, select Branch.

3. Run the Add Rule or Import Rule geoprocessing tool.