Cause

No plug designation is assigned at a bus port.

Solution

1. Locate bus port in the schematic using the Go to (graphic) functionality that is provided by the popup menu of the Message management dialog.
2. Open the property dialog of the bus port.
3. Bring the first tab to the front.
4. Enter the missing value in the Plug designation field.
5. Click [OK].
6. If required, start a new check run.