how do I extract the following values I'm watching Rigging's tutorial. I'm having trouble applying the driver to the transform constrainted  bone. The following words should be included in the path value. I don't know how to extract the following values.

pose.bones["upper_arm.fk.L"]["isolate_rotation"]
When choosing rig as target object, typically you can select desired bone.



Be sure your 'rig' object is of 'ARMATURE' type.