I was simply replacing a existing dimmer switch for my kitchen lights with a newly bought Feit electric smart 3 way dimmer switch. After studying, I have located where the dimmer switch should be replaced, the load side, as written on the instructions it came with (rather than the line side).
After rereading the instructions, I finally understood what I missed because the problem was the new dimmer would work only if the switch on the other side was turned on. It was that I did not connect the line wire (common wire) with the traveler wire as instructed by the instructions. I simply didn’t think I had to modify the configuration on the other switch. (I thought well the original dimmer switch was working, so if I wanted to make it a smart dimmer, all I had to do was replace the dimmer with the new one.).
Anyway it all works perfectly now, meaning the lights can be turned on or off on either the dimmer side, or the switch side.
My question is, in order for my switch to work, I had to clamp the traveler wire to the switch using the side screw as the line wire was already plugged into the back and there was no second hole. And because both wires are 12 gauge, I couldn’t jam them both into the hole in the back. So all I could do was leave the line wire plugged into the back and I clamped the traveler wire to the side and screwed it in. Is this safe?
UPDATE: Thank you for all your help. I ended up taking all of your advice and fixed the switch! I found that the previous person who owned this house, or the previous electrician just did some questionable things after learning from you all! Here's the final image: