How I can change
Transform Orientation with the keyboard in Ubuntu? In some linux systems (and many others) Alt + Space are a system combination to show application menu and are captured by the OS.
Personally I'd change the desktop manager binding from alt to super, but in Blender you can change it by going into User Preferences > Input. Then type in the search box "Orientation". You can click on the "Alt Spacebar" field, after which you can give the wanted key combination for Orientation Selection.