Trying to get AI to walk up walls for the first time

So I'm really happy at finding out there's supposed to be a simple solution to having enemies that walk up walls and that you can download a Unity package which will let your agents go up walls without any issues. I've got a classic bug enemy that I want to put into the engine and have wonder up walls and ceilings, however I've already run into a problem.

https://unity3d.college/2017/05/09/ceiling-wall-navigation-unity3d/

I found this tutorial which all makes perfect sense, but first problem? The engine simply doesn't let me create multiple NavMesh Surfaces despite me going through the tutorial. I can create a standard one of course but it's only on the floor and I can't move it at all to match up with my wall.

Would somebody mind explaining what's going on here? I'm sure it's going to be something dead simple like me not enabling some option or other to allow me to fix everything. I just double checked to make sure I wasn't being stupid and rotated my wall back to default and attach a NavMeshSurface to it and it was the same result, it just defaulted to creating one on the floor.