Hey i'm with the 2.79 version.
Here's the solution to fix the problem:
If you only have one vertex, and try to add the modifier, the error will appear.
I think blender is looking for at least 2 vertex to defne one as a root and the other one as the extruded vertex.
So:
1°) Make sure you have the vertex mode active !
2°) Add a single vertex in object mode, go in EDIT mode and extrude it.
3°) Add the skin modifier.
You're first vertex will be the root.
Or:
1°) Make sure you have the vertex mode active !
2°) Add a single vertex in object mode
3°) Add the skin modifier.
The error will be here but just extrude the vertex and it will goes off.
(NB: the extuded vertex will be the root and will follow your extrusion.)
