Error compiling pixel shader – Arma 2

You're probably running the Official ArmA 2 Beta modfolder named "beta", while you did not adjust your shortcut appropriately e.g:

"C:\Program Files\Bohemia Interactive\ArmA 2\arma2.exe" -mod=beta

should be:

"C:\Program Files\Bohemia Interactive\ArmA 2\beta\arma2.exe" -mod=beta

Or remove beta from the -mod= parameter