Cause

Although identified as relevant to safety, the Safety address property does not have a value at a PLC box.

Solution

1. Locate the PLC box 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 PLC box.
3. Bring the first tab to the front.
4. Enter the desired value in the list of properties in the Safety address field.
5. Click [OK].
6. Then start a new check run.