Cause

The source or target of a connection of which several representation types exist in the project is not available in one representation type. For example, the target of a routing connection of the "3D mounting layout" that exists in the schematic as a multi-line connection has been deleted from the layout space.

Solution

1. Locate the connection using the Go to (graphic) function from the popup menu in the Message management dialog.
2. Find the missing source or missing target via the existing connection.
3. Place the missing source or missing target.
4. Then start a new check run.