I think it has been since the last Unity update that the sky textures have been missing except for a small arch near the horizon in whichever direction I'm looking. I have tried adjusting my video settings from high to low quality, with no impact on the missing sky texture. I am running on MacOS 10.12.6, 3.7 GHz Quad-Core Intel Xeon E5 processor, 16 GB 1866 MHz DDR3 RAM, and AMD FirePro D300 2048 MB graphics card. Not top of the line, I know, but the game runs just fine expect for the missing sky.