Unable to cap object

I’m trying to cap the edges of this shank but it keeps saying “The openings did not have closed, planar loops of edges”. Can anybody tell me how I can fix this?

Rhodlite Stock Ring.3dm (3.7 MB)

CAP works on planar edges, ie. if you take a flat plane and use it to cut your object you end up with a collection of planar curves. Your model isn’t planar with those extra pieces sticking in & out…

thank you!

You can create much cleaner geometry for this with Revolve: