Hi Sam, at about the 4 minute mark in lecture 43 (Materials - Standard Shaders 3/3), you are trying to create a copy of the ‘Glass 1’ material in the project window. Now, you are right in that there isn’t an option from the menu that appears when your right-click the material, but there is a way to duplicate it.
If you press Ctrl-D on a Windows PC, the material will be duplicated, and I assume there is a similar keyboard shortcut on a Mac as well. It is a bit strange that duplicate isn’t listed in the right-click menu in the Project window, when it is in the Hierarchy, but that’s the way it is!
Hopefully this is useful to some of you!