When you will have a good training of Helper Lines
you will can open the little white trapeze for more options! ;)
Who appears when you release the mouse button drawing an helper lines!
Alas don't appears on my recording system! :(

If you rotate the edges on the two balls to be away from the fillet then the fillet will be successful (at least in MOI5 beta)

Thanks for your answer Pilou.

There was a small mistake on my part.
The circle was not exactly round. Therefore, the center was not exactly in the middle of the circle.
I knew that with the helplines. ;)

Thanks anyway.

Yes the curves generator generate some troubles! It's a good habit to separate them!

You can also use the script of Max Smirnov for find the Center of Gravity! ;)
(just make some thickness to your surface! ;)

https://moi3d.com/download/scripts/max/CVolume.v.3.0.2022.05.24.zip