Cause

You created a pin without a plug definition in the Plug Navigator or placed it in the schematic.

Solution

Possibility 1

Select Popup menu > New in the plug navigator.

In the expanded Selection tree of the subsequent dialog (via Electrical engineering > Terminals and plugs > Plug definition > Plug definition ) navigate to the desired function definition (Plug definition for female pins, Pins or Male and female pins). Click [OK], and assign the displayed DT or the plug to the plug definition in the Properties <...> dialog.

Possibility 2

Under Insert > Plug definition place a plug definition in the schematic and assign the displayed pin DT to it.

Refresh the connections and then start a new check run.