Both these functions stopped around the same time. El Capitan, but I think these worked after the upgrade and now don't. Both functions work in other apps. Just not in Finder. Repaired permissions on MacHD, checked and unchecked the command for spotlight in System Preferences>Keyboard>Shortcuts>Spotlight. Using command+space in Finder gives the error chime.
Solved by resetting NVRAM. Power down. Restarted holding down command+option+P+R until I heard the second chime. Released all the keys. Now both functions work in Finder. Command+Space brings up Spotlight search box. Command+Tab brings up the list of apps to toggle through.