[Rigging] Can’t attach weapon to character’s hand

Hi there, I’m quite new to Unity. My goal is to get the M1911 hand gun attached to the character’s hand, However I got these errors:

Could not resolve 'rig_layers_hand_ik/right_hand_ik' because it is not a child Transform in the Animator hierarchy.

And:

 InvalidOperationException: The TransformStreamHandle cannot be resolved.   

Is there any solution to solve this? Thanks!

By the way, the project package link: https://drive.google.com/file/d/1q4_3qSCXwkuUj3svr07Gyz6Bv0Tkc797/view?usp=drivesdk