Hi Chipp - to get end caps on extrude the curve has to be both flat and closed. Your curve here is flat, but not closed - if a curve is closed its object type in the upper right corner of the properties panel will read as "Closed crv", yours just says plain "Curve" which means it isn't closed.

To find out where it's open, turn on control points and squiggle them around a bit, when you get a spot that tears open that's the opening, in this case it's here:





If you align those points so they are collapsed together, then you should get an end cap when you do an extrusion.

- Michael