Custom Shortcut Not Working Ubuntu 21.10
Solution 1:
I added the English (US, euro on 5) input source to put the Euro sign over the 5 key.
Settings -> Keyboard -> Input Sources
After selecting English(US), choose the layout, in this case English (US, euro on 5).
Solution 2:
Ctrl-Shift-U 20AC will print a € symbol. It works for any unicode symbol, actually. After you press Ctrl-Shift-U, an underlined U will appear, type the 20AC Unicode and then hit the space bar. Voila, a € symbol will appear.