In the Object Placement example scene, you can modify line 23 of the PlacementController.cs script to exclude touches that are over a UI element.
The Touch class contains a method named IsTouchOverUIObject() which returns a true if the touch occurred over a button, label, image, etc. in the canvas.
As published, the if-statement on line 23 appears as…
if (touch.phase == TouchPhase.Began)
…and we can modify it to appear as follows…
if (touch.phase == TouchPhase.Began && !touch.IsTouchOverUIObject())
…in order to only continue calling the enclosed TouchBegan(touch); statement if the IsTouchOverUIObject() method returns false.