How to emulate a right mouse button click on touchscreen?
Solution 1:
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.
Solution 2:
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.