Major Bug Fix:
Now works with Blender 4.0 changed it to use blender 4.0's method of overriding context through here: https://docs.blender.org/api/current/bpy.ops.html#overriding-context
To install right click on "Fuse_Skeletons_v.X.X.X.zip" file right below THIS description > Save Link As to download it, do NOT unzip it. In Blender Edit > Preferences > Add-Ons > Install > in the file explorer find "Fuse_Skeletons_v.X.X.X.zip" and select the zip file pressing Select > in the Add-Ons search search for Fuse Skeletons and enable the Add On to complete the installation.