Hi Steve, end caps are not created for partial revolves, they're only created for holes that are simple closed planar loops as you get with a full revolve.

Your result looks like this, shown here with the hole area highlighted in yellow:



The problem is that the hole to be filled has a closed outline but the outline is not all on just 1 single plane, it's on 2 different planes and the end capper does not currently know how to automatically divide things like this into individual planes.

So in order to cap it you need to divide it yourself - draw in a line here: (or use Edit>Separate on your original shape to get that line):




Now once you have drawn in that 1 dividing line, you can select it and also these 3 other surrounding edges like this:



With that selected you can then run Construct > Planar to build a planar surface there.

Then make a selection like this on the other part:



Then run Construct > Planar to build a second planar surface for that area.

Now you've got 3 surface pieces (the revolve + the 2 planes just created), select all 3 of those and run Edit > Join to glue them into a solid, I've attached a 3DM file with that result in it.

- Michael