Never had this problem before, how do i fix it?

Have you tried uninstalling and reinstalling the unpack?

reinstall portal steam

• Bitrain

reinstalling portal on steam didnt work trying reinstalling the unpack now

I know what the error means but I don't know what causes it, so I can't really tell you how to fix it unfortunately. There's different reasons for it to happen but I don't know what the fix is for it.