Hi Oskar - the regular fillet command for curves expects the curves to be planar to one another. Your curves do not fit all in the same plane so that's why you can't fillet them.

Also the regular kind of fillet solution is an arc piece of a specified radius, which is tangent to both of the curves. But when you have curves that are bending around in 3D and not on the same plane, it is easy for there to be no such co-tangent arc with that radius that exists.

One other method you can use instead is to trim back some space between the curves and use the Construct > Blend command to put in a blend piece between them, like this:






To trim back the pieces, you can select them and use the Trim command, and push the "Add trim points" button which will let you click a spot on the curve for the location to cut it, or you may want to draw in a sphere where they touch and use the sphere as the cutting object in the Trim.

- Michael