@Garrett617 So there was no Neutral available in the other box? 

Can you tell whether the location in your images - is it the Line or the Load that is connected there in that box? 

It would have to be the Line in order for the HS210 to work correctly in that location. 

(disconnect from the common and momentarily turn breaker back on and check for Line voltage)  

@Garrett617 

p.s. It's hard to tell - from second image it almost looks like the red and black (one on gold screw, other on the black) are coming from same Romex - is that the case? Or just the image creating illusion as such? 

If so, then that would have been wrong - the black screw is the common - that would have to be the Line (or the Load, depending on which box is which) 
And in the last pic it looks like the black on the other gold screw (the side with only one) goes to a nut with two other black wires; that would appear to be either the Line or the load (so again, if so, it should be on the black screw) 

I'm speculating here, but do you have two light fixtures? Is that perhaps why that one black wire goes to two others? would that possibly be the load? There are separate Romex cables there on that right side of the box that appears to connect to that. 

Either way - even if it's a Line, then it would still be wrong being connected to a traveler terminal  

@Garrett617 So is my prognosis correct in that the load is those two wires come together and go to that single wire that goes to screw on the single screw side? 

Be careful but - 

identify which is line voltage on that other switch

Turn off breaker 

Disconnect that pigtail wire (coming from the two blacks) from the switch 

Turn breaker back on 

Test for voltage at that pigtail - if none, good chance those are the Load Wires 

Next turn back off

Connect to the line of that other switch 

Turn back on 

If lights indeed light up, then you have confirmed they are the load wires

And if that is the case, then you cannot use the HS210 here as when using ONE it must go at the line end 

(either way, that switch was definitely not wired correctly to begin with!)