Cause

An air-conditioning field that has already been assigned was found at an enclosure in the layout spaces of the project.

Solution

1. Locate the enclosure whose air-conditioning field is not defined uniquely in the project using the Go to (graphic) functionality from the popup menu in the Message management dialog.
2. Call up the properties dialog of the enclosure and change the value in the Power dissipation: Air-conditioning field field. Ensure that you select a number that has not been assigned to another enclosure in the layout spaces of the project.
3. If required, start a new check run.