From the image on the left (view can be expanded from link below), you can see that the control status is "Offset resistance not valid ". This means that the control does not know the value of the weld gun resistance that needs to be substracted from the resistance reading to result in the dynamic resistance of the metal piece being welded.

Beside the "Weld Mode" button, there is a button that states "R Measure is OFF". Press this button then you will get a confirmation request to turn ON R Measure