Problem with the gripper pybullet

[Yangziqi1920]

Hello
I am working with pybullet , I want to make a simulation of the dobot magician robot, but I have a question about the gripper of my robot, I want to use constraint th make my gripper grisp the object, but ie seems not good.

look this picture, the gripper cannot close.
I don't know what is the reason, this is problem of program or the urdf file.