From what I’m seeing here you are already parenting the hair to the dummy, making the hair already be a part of the dummy and making AddAccessory error out since the accessory is already in a character.
Oh, I thought AddAccessory was some function that aligns all the CFrames together.
How do I simply add a hair mesh or hat to a humanoid? Do I need to manually do all the CFrames/welding? I thought that was what this AddAccessory function was for, to do that for you.
Nope. You can give a character an accessory by either parenting it to the character or by using AddAccessory(). And yes, it does put the hair automatically IF the handle and the head have the “HairAttachment” instance inside of them.