I'm trying to wire the HS210 to one of the two switches in my parent's master bedroom and leave the other dumb switch intact. I'm running into an issue where the most I can manage to get is rapid flickering out of the lights when I flip the dumb switch on, and, the HS210 doesn't seem to be working at all. I can't figure out what I'm doing wrong. I thought I knew this stuff pretty well as I re-wired all the switches in my house, but, my house is much newer than my parent's.

The "light" in the bedroom is actually a half receptable where a lamp is plugged in. The first switchbox has a 2-wire romex plus a 3-wire romex. The second switchbox only has a 3-wire romex. If I'm not mistaken, my common wire is the one coming out of the 2-wire romex in the first switchbox. The only switchbox that appears to have neutral wires is the first.

First switchbox:

Assuming I have labeled everything in that picture correctly, I connected everything to the proper locations on the HS210 (white into the neutral bundle, ground to ground, common to common pole, and the two travelers to the top poles). I didn't take a picture of the second switchbox (dumb switch), but it only has a red and a white wire connected to the traveler poles and a black wire connected to the common. The wiring diagram seems to align with this one I found online, except, the only difference is that this controls a full outlet instead of a half outlet:

Here's the part I'm confused on: I checked the "common" wire in the first switchbox with my multimeter by touching the leads to it and the ground wire, and, it was not reading 120VAC. It was reading a small amount of voltage, but, not what it should have been. So I guess if that's not the actual common wire, where the heck is it? Is this perhaps some janky setup where the common is actually coming from the outlet wiring instead of the switch wiring?

Thanks for any help you can offer.