Why is this not closed?

Why doesn’t this close up, to allow me to make a solid object?

Test.skp (15.5 KB)

SketchUp isn’t smart enough to make a surface from your border lines, Sketchup only makes faces from edges that intersect on a plane, so you’ll need to make the faces yourself and then soften the edges. See the attached example

Test filled and smoothed.skp (32.1 KB)

Thank you