Hey, thank you for the feedback. You need to pull a frame other then 0. Frame 0 is the "Zero frame" and that frame should only have the rest pose. So that is why I print that error when you try to pull that frame, to prevent you accidently posing your character in that frame.
When you click "Pull" it tries to pull the frame that the blender timeline is currently in. So to fix that problem you should just move the play marker from 0 to some other frame and pull again.