Hi Will, MoI can only extrude just a single selected surface with the extrude command. It doesn't attempt to extrude a selected object if the object is made up of multiple surfaces. This is to avoid stuff like trying to extrude something like an entire box which would make a mess.

That's what you've got in this case - your shape here is actually made up of 5 smaller surfaces that are joined by their edges to make the larger loop. You can break this into individual surfaces by selecting the joined surface, and then doing Edit/Separate. After you do you can then extrude the individual surfaces, but that will result in 5 different extruded objects.

I'd guess that you created this surface using Loft (or possibly sweep along the short line) (Edit: now I saw your video and saw it was sweep)? You probably should avoid that for these types of totally flat shapes, instead use Construct / Planar, which will build a single trimmed planar surface out of those curves.

And actually if you're going to be extruding it, you can skip building the face entirely because extrude has the equivalent of Planar already built into it. Just select the outside and inside curve and run Extrude directly - it knows to make holes for curves nested inside each other.

Let me know if you need any more details!

- Michael