You may simply need to be in a sketch, either 2D or 3D, to use this function.. (i.e. between two swmodel.SketchManager.Insert3DSketch(true) calls). I've got this working but am having issue getting it to make non-closed curves. The option exists in the
interactive tool, but there's no documentation for it in the API...I'm guessing it may be the mode setting, but nothing on which value to use. p.s. I realize this is a 15 year old reply, so hopefully it helps someone out there.

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For full context, visit https://www.polytechforum.com/solidworks/solidworks-api-makestyledcurves2-59661-.htm

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