I made a Python add-in for ArcMap, and one person is having trouble with it. They are using ArcGIS Desktop 10.1 SP1.
They get the following message:

What exactly does this mean? No one else who has used the add-in has had a problem. Could this mean that Python is not installed on their machine? I did a few searches, but couldn't find what exactly causes this error.
Thanks for any input.