Solid Inspector showing Surface Borders error

I’ve drawn a square (10m x 10m) and Solid Inspector is showing
1 Surface Border error.
I’ve come to this from trying to draw a more complex shape and still getting the warning.
Square.skp (63.8 KB)

A single flat face is not and cannot be a solid, so it is telling you that all four edges of your selection do not connect to another edge.

An square isn’t a solid, you must have a cube or geometry that has width height and depth, it must be closed and have no stray edges in it.

Duh yea right. Silly me.
Thanks for you response.