Well, what does the error say?
My best bet, not knowing what the error says, is that you are getting a nil error, as @unnatural_skills is undefined. That would be the case if any of the scripts you are using (probably Multi-Slot if it does what it name suggests) overwrite the setup method of Game_Actor and Skill Teaching Equipment & Items is above it in the Script Editor.
So, try putting the Skill Teaching Equipment & Items script below all of the custom scripts you are using in the Script Editor (but still above Main), so put it below the scripts you just mentioned.
EDIT::
Though, thank you for calling my attention to this script - I realized there was a pretty serious error when an actor learns a new skill that he currently has through equipment. Updated the script to 2.0b!