Head Not Attaching To Body

Hi all,
I'm making a game with Thanos in it. I've been trying for a very long time to get my head to attach to the rest of his body. I just realized, though, that the head was in another element in the hierarchy. I put the head from one game object to another, but then I tried to save and it didn't work. The changes in the hierarchy weren't saved, but the changes in the avatar were, so it left me with a nasty error.

What I want to do


Please help me fix this!

DOWNLOADS
Click here to access the folder, and here's the unitypackage

Raw Links
Folder
https://drive.google.com/drive/folders/1-38n7WL8q0iBWPmXL6vwq-0EqqYBM0LV?usp=sharing

Unity Package
https://drive.google.com/file/d/1Q73wFBEMzNPS5XzLSQhlHPuA5bedC7IP/view?usp=sharing