Hey there, thanks for reaching out!

It’s hard to be sure from your screenshot alone, but generally, endpoints can only be applied on Vectors with two or more endpoints. Technically, Figma doesn’t consider an Ellipses shape as a Vector with multiple endpoints. Because of this, you’ll need to instead select each corner of your shape and adjust the corner radius (see here).

We understand how this might be misleading, so we’ll also pass that feedback along to our product team!

I’d consider different, mire sophisticated method to achieve what you need. Have a look at the arc tool - https://help.figma.com/hc/en-us/articles/360040450173-Arc-tool-create-arcs-semi-circles-and-rings

You can make multiple circles, each is an arc. There is a simple math to calculate the distance between slices, just set the starting point and the sweep to set the length of the slice. Next slice starting point is a sum of the previous slice starting point + sweep + gap between arcs.

Once you are happy just combine shapes with Boolean and you are good to go