I have a problem whit the extrude feature
Nearly same solution like above.
Delete Sketch 6 - unnecessary
Just delete Curve2
Delete 3D Sketch
Edit Sketch 5 to have it symmetrical (for the nice shape)
Use the Sketch 5 to create spline line
Use the spilte line to create Offset Surface (20 mm is for example)
Use the Cut Thicken to have it removed, that's it...