Hi Mala, the main goal of curve filleting is to make a circular arc tangent between 2 existing curves.

Since a circular arc is a planar curve, it is expected that the 2 curves on either side of it should be planar curves and on the same plane.

Your case fails because it does not match that criteria, each of your curves is planar but they are each on their own separate plane, not both on a common plane.


Another method you could try would be to draw a sphere at the juncture between 2 curves, use the Edit > Trim command to cut the curves with the sphere so that there is some space opened up between them, then select the 2 ends and run Construct > Blend to put in a blend curve in that open space.

- Michael