Hi,

It looks like you have trimmed a sphere to create the surface you show. The control points will show the underlying untrimmed surface (the sphere)

Due to the way the sphere as been trimmed, there is a seam crossing the surface which can give problems when you make an extrude and produce a non-solid which will not then fillet. I would suggest that you trim a sphere in a way not to include the seam, (you could continue with what you have, but would need to make further steps to sweep around the surface).

so, it looks like you have trimmed a sphere:-

with control points on, it looks like:-


When you extrude such a surface, you can have a problem due to the seam (the line that crosses the surface)


If, before you trim the sphere, you rotate the sphere so the seam is not included in the surface you want to extrude/fillet:-




You will then be able to extrude and then fillet.