ar remote: 'ARExtensions' does not exist in the namespace

I want to use xr.remoting package in my project, but I got an error :
Library/PackageCache/com.unity.xr.remoting@0.0.1-preview.10/com.unity.xr.mock/Runtime/XRMockCameraExtensions.cs(2,22): error CS0234: The type or namespace name ‘ARExtensions’ does not exist in the namespace ‘UnityEngine.XR’ (are you missing an assembly reference?)

I use Unity 2019.2.0b7

Hie…
How could you solve this ?

Not yet, Unity team side it not work well now, so I give up.