Generally the wire label option is greyed out any time a wire is not connected correctly on both ends. No idea why both ends of a wire need to be connected to something in order to display color and gauge, but that's how SWE works. Not sure why it would be different between 2016 and 2018 though. Best suggestion is to delete the wire and draw it again. Possibly delete the things the wire is connected connected to and add them back in again if nothing else.
There are equipotentials and then there are wires. When you click the draw wire button, you are drawing equipotentials actually. Only when an equipotential touches exactly two connection points have you created a wire. You can add equipotentials labels at any time, but you can only do wire labels once a proper wire had been created.