From GitHub: https://github.com/Unity-Technologies/InputSystem/issues/332

When using UI, the "touch [Touchscreen]" binding option does not bind the Action to any input.
When examining the actual binding path, it is "<Touchscreen>/touch". However, there is no Control in Touchscreen device is named touch, and probably the reason it doesn't work.
The workaround is to manually put in "<Touchscreen>/<touch>", and bind the Action to all touch input detected.

Repro:
1. Download package and project from: https://github.com/Unity-Technologies/InputSystem
2. Go to Assets -> Demo -> SimpleDemo
3. Open SimpleControls Input Action UI
4. Select "fire" in the middle section, then change Type in the right section to "Any" and check "Pass Through"
5. Change the Binding "PrimaryAction" to "touch [Touchscreen]", and remove all the Interactions
6. Save Input UI and close
7. Run it on a device with touch support and observe the scene.

Workaround:
8. Select the touch binding just added
9. In the right section, click "T" button next to the Type drop down menu.
10. The drop down menu turns into a text field, put in "<Touchscreen>/<touch>" instead.
11. Try to run it again

Actual: "touch [Touchscreen]" option in the UI doesn't bind to any input.

Note: Rene said in the spread sheet he has a fix but needs to be implemented. Not sure if it is in place, but the bug still happens.