Flex - [Error] [carb.gym.plugin] Function setGpuContactsRequested in not implemented

HI, I got error when I add this code on the while loop

while not gym.query_viewer_has_closed(viewer):

    gym.refresh_net_contact_force_tensor(sim)
    contact_tensor = gymtorch.wrap_tensor(gym.acquire_net_contact_force_tensor(sim)) # THIS RETURN ERROR

    gym.simulate(sim)
    gym.fetch_results(sim, True)
    gym.step_graphics(sim)
    gym.draw_viewer(viewer, sim, True)
    gym.sync_frame_time(sim) 

[Error] [carb.gym.plugin] Function setGpuContactsRequested in not implemented

Where does this error affect? What should I do to solve this?

Thanks!

++ Comment added
I checked this error occurs only for the FLEX engine… I use some deformable object and rigid-body gripper. I want to know about forces on the each jaws of the gripper. Could you recommend me what is the best way to measure forces?