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Hi, I'm Mike and I'll be assisting you. Please stand by while I review your question.
What are you using for a tester?
That what I thought. 3 way Z-wave switching required the Z-wave to be installed where you have the switched hot to the fixture, a hot wire and a traveler wire. If the one switch shown in your diagram has all3 wires in one cable, you'll be short one wire for the installation. That indicates the feed is at the fixture and not at a switch box. What you should have is 2 blacks and 1 red on the old switches.
You need a 2 wire voltage tester to positively identify what is hot. Noncontact testers only alert you there is a hot in the box, one hot will faultsly show other wires as hot when they aren't.
The red is to the light and the red/yellow is the only traveler. It uses the single traveler and a neutral at the other switch. So at the Z-wave you need a hot, neutral, traveler and load wires.
At the second switch, the white neutral wires to the black screw. The traveler wires to either gold screw. One gold screw is vacant.
Very good, you're welcome.