The problem is the Property ID type. You need object and you have Armature. The full path is bpy.data.objects[“Armature”].pose.bones[“Bone”][“prop”] as you can see it’s bpy.data.objects not bpy.data.armatures. In the driver editor click the dropdown(Surrounded by yellow box in image) and select objects. Then select Armature in the object dropdown next to the icon and paste your path.
