How to emulate a right mouse button click on touchscreen?
You need to enable the secondary click (as AliNa commented) with gsettings set org.gnome.desktop.a11y.mouse secondary-click-enabled "true" or with dconf editor.
Then it is possible that Ubuntu handles the touchscreen partially like a touchpad, where a touch does not trigger mouse-press. Instead you need to short-tap + touch-and-hold.
Just keep touching the screen for 2 or 3 seconds and what is called "right menu" should appear. This is the normal way to emulate a right click on any touchscreen.
If you have a Wacom touchscreen (using driver xf86-input-wacom) and it is capable of at least two-finger multi-touch, then the following will produce a right-click:
- Tap-and-hold with the first finger.
- Now tap with the second finger.
If it does not work yet, find out the product name of your touchscreen from xinput --list and then enable this feature with:
xsetwacom --set "[your touchscreen product id]" Gesture on
To make this change permanent, see my other answer for the technique.
Unlike the long-press right-clicks under Gnome / Ubuntu Shell, this works in all desktop environments because it is a (legacy) driver-level feature. Just be aware that it may cause issues with left-clicking by touchscreen in some programs, which is why I had to disable it on my system.