Cause

A net definition point was placed on a connection and given a potential name. The potential is not defined anywhere in the schematic.

Solution

Possibility 1

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

Possibility 2

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

Refresh the connections and then start a new check run.