So you’ve installed Ubuntu 18.04 LTS and can’t change keyboard layout to Alt+Shift or Ctrl+Shift? And when you trying to edit configuration in keyboard setting from Super + Space nothing happens?
You can do it other ways but easiest way is to use Gnome-Tweak Utility.
Install it from apt from your terminal
$ sudo apt install gnome-tweaks
Then launch it by typing
In Tweaks app, locate tab “Keyboard & Mouse” and press on button “Additional Layout Options”
In drop-down menu find “Switching to another layout” and choose prefered option to layout switching, for example Alt+Shift.
You’ve done, now check if it works. If not – please post in comments and we’ll try to help.