Not a solid

Hi Community
I have a simple surface that I want to cut out of a solid, but it keeps coming up with the ‘not a solid’. I have run Solid Inspector 2 and CleanUp without success. SI2 says there is a border issue. I have redrawn it, but with no success. Can anyone help please?
surface.dae (7.1 KB) surface.skp (107.3 KB)

A single surface cannot be a solid, it needs thickness.

Make solid

Thanks Box. That makes perfect sense!! Cheers