What is zero thickness geometry?
Zero thickness geometry is when a feature is created that can not be solved due to the fact that there is precisely zero material at only a point or an edge. An example would be if you created an extruded circle that is tangent to an extruded square. There is a good explanation here: http://help.solidworks.com/2012/English/SolidWorks/sldworks/HIDE_ZeroThickness.htm
I've reposted the general guidelines from that site below as reference.
Answered with a tutorial: https://grabcad.com/tutorials/what-is-zero-thickness-geometry