Solution 1:
You are in the right place, but the wrong selection.
Select "Spotlight" from the list on the left. You can then uncheck the shortcut for Command Space allowing you to assign that to the shortcut you want to.
You are in the right place, but the wrong selection.
Select "Spotlight" from the list on the left. You can then uncheck the shortcut for Command Space allowing you to assign that to the shortcut you want to.