Hi John,
It is most likely that the script no longer works with the version of Blender you are using 🙁
(I am not the maintainer of that script)
The easiest way to do this manually would be to lock the groups you want to keep (you can select the bones in the viewport and hit K to bring up the weight paint locking menu)
Lock the ones you want to keep, and remove unlocked groups.
And don’t forget to unlock them again.
I’ll see what I can do to get this implemented into Blender itself.
Seems doable. (it just won’t help you right now)
