Thank dude^_^
I think I made a careless mistake~
I disregard the coupling option. Because I use "Tangency then curvature" this tab, thus I can't do it work out.
Anyway thank you again!
Several coupling types are available in the Coupling tab:
Ratio: the curves are coupled according to the curvilinear abscissa ratio.
Tangency: the curves are coupled according to their tangency discontinuity points. If they do not have the same
number of points, they cannot be coupled using this option.
Tangency then curvature: the curves are coupled according to their curvature discontinuity points. If they do not have
the same number of points, they cannot be coupled using this option.
Vertices: the curves are coupled according to their vertices. If they do not have the same number of vertices,
they cannot be coupled using this option.