Perhaps this is a problem with Unity 2019.3, but this section of the course didn’t work for me. I took forever trying to fix it, and then went on to the next section - where I had the same problem!
However, @JimmyB in the Discussion fixed this:
Creating A DeathHandler Class?
The problem I think is that the Input.GetButtonDown in Weapon.cs (using some form of OnMouseClick function) is over-riding the mouse click on the Canvas.
So even if you put a debug/print message to respond when clicked, the Weapon would shoot instead. Very puzzling. Anyways, the solution above works (and removes the weapon image, which is cool)