Hi @yotam, on order to use eye-tracking with the HTC VIVE PRO, you need to use the XR Unity SDK whilst it appears from your code you are using the Unity SDK for Desktop API.
You can download the appropriate SDK for your hardware @ https://vr.tobii.com/sdk/develop/unity/ which comes with the appropriate tutorial and getting started guide.
Please go through this and let us know how you get on. Best Wishes.