A non-portalable surface is a wall, ceiling, or floor that you can put portals onto by using the Aperture
Science Handheld Portal Device or the Aperture
Science Quantum Tunneling Device. It appears in many games from Valve (some
legit, some obtained using mods, and some impossible to obtain), like Half-Life 2 and Portal.
I couldn't put the exit portal, because it was a
non-portalable surface! You
don't even know what the white gel is made of! I have got to seriously quit co-
op.