I had the same problem and lost a couple of hours trying to resolve it.
I found a solution, based on what can be found here:
https://help.ubuntu.com/community/ComposeKey.
This is what you need to do:
In the terminal type:
sudo gedit /etc/default/keyboard
After typing your administrator's password, the text editor will open. Find the line that reads:
XKBOPTIONS=""
Enter the desired key value between the quotation marks. In my case it was compose:lwin – to map the left Windows key as compose. In your case it should be: compose:paus. So the line should look like this
XKBOPTIONS="compose:paus"
A list of possible options can be found when you type
grep "compose:" /usr/share/X11/xkb/rules/base.lst
in the terminal.
Log out, log in again – should work just fine.
(PS: Why they turned off the possibility to customize this easily in the system settings, still remains an unresolved problem.)
You will need to install dconf Editor from the Ubuntu Software Centre so it will appear in your applications.
If you use an applications list it appears under Sundry or if not in your Activities Overview (type dconf in the ~type to search~ box).
There are other ways of achieving your aim but this is a simple gui method not needing terminal skills.
$ dconf write /org/gnome/desktop/input-sources/xkb-options "['compose:{key}']"
where {key} is your chosen key-name (e.g. "['compose:ralt']" would make right alt your compose key.
Best Answer
You may use (GNOME) Tweaks to set the option.
First install Tweaks by running
(or
sudo apt install gnome-tweak-tool).Alternatively, for those who prefer the GUI:
launch the Ubuntu Software application.
search for "gnome tweaks".
install.
Then launch Tweaks and go to "Keyboard & Mouse" section. The "Compose Key" options is disabled by default. Click on the "Disabled" button.
Then a window will pop up. Toggle the switch at the top on and then you'll be select the compose key