Question: I'm trying to enable my polystrips addon but I receive this error every time I try to check the enable addon box.[![enter image description here](https://i.stack.imgur.com/04VOC.png)](https://i.stack.imgur.com/04VOC.png)

Answer: 
It appears you need to grab the code from here <https://github.com/CGCookie/retopology-lib> and put the source files (ie `__init__.py, common_utilities.py, common_drawing.py, common_classes.py`) into the empty retopology-polystrips-master/lib folder
Most commonly, these kinds of errors have been solved by disabling any old versions of the addon, deleting them from your addons directory, re-installing the addon, saving user preferences and re-starting blender.

best,
Patrick