I'm trying to insall a three way dimmer switch to replace a dumb three way dimmer. This is in the same box as an existing single-pole dimmer, also from Kasa that has been installed previously and everything is working fine.

The prior installation has the smart single-pole dimmer (Kasa) on the left and a dumb three-way dimmer on the right, with the black line (load) shared amongst the two:

Trying to install the new three-way dimmer (main one from the kit), I also shared the load, just like before and also connected it to the neutral. Even without connecting the travelers, the switch receives power, clicks a few times and then starts flashing red. It does the same thing when I press the right button to restart the switch. Same thing happens if I connect the two travelers.

Any Ideas of what I could be doing wrong?

Thanks.