Citizen Hand CAT Rig problems

I’m having a odd issue using the Citizen hands rig I got from here

I can’t link any type of constraint with it, for example I want to animate a shotgun but I want to the IK Target to move when the model moves, I’d simply link a constrait from the IK controller to the model but this is impossible as any attempt at doing it results in “Link Constraint Not Completed”

Here is a picture