Cause

In the schematic at least one non-connected symbol connection point was determined at a pin.

Solution

1. Locate the relevant pin definition in the schematic using the Go to (graphic) function from the popup menu in the Message management dialog.
2. Connect all open symbol connection points of the pin with a component so that autoconnect lines are formed.
3. Or, if the pin is not required anymore, delete it from the schematic.
4. If required, start a new check run.