Did you follow the steps from the User Manual? https://tobii.github.io/UnitySDK/manual#getting-started
Try importing the package into a blank project.
Could you please record a video showing how you import the package and run the demo scene? And show where the error comes from in the code?
We have different SDKs for different frameworks. If you are using Unity then Unity SDK is the way to go.
Here you can find a sample of how to write gaze point to the log in Unity – https://tobii.github.io/UnitySDK/scripting-api#tobiiapigetgazepoint