Cause

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

Solution

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