Cause

A net definition point was placed at a connection and given a signal name. The signal is not defined anywhere in the schematic, however.

Solution

Possibility 1

Enter the name of an existing signal definition for the net definition point.

Possibility 2

Define a new signal in the schematic using a potential connection point, for example. Give this connection point the potential name and signal name of the net definition point.

Refresh the connections and then start a new check run.