AsyncGPUReadbackRequest could not found

I am getting errors like "The type or namespace name 'AsyncGPUReadbackRequest' could not be found (are you missing a using directive or an assembly reference?)" "The name 'AsyncGPUReadback' does not exist in the current context" Although I have included using UnityEngine.Experimental.Rendering. Can you please tell me how to resolve this issue? I am using Unity 5.6.4p4.