@mayoketchup said:
I noticed it doesn't work with native lines only with freehand and arcs.
I downloaded your bezier spline plugin and now it works perfect with these curves 👍 .
To create a Sketchup curve out of a sequence of edges, you need a Plugin, as this is (still) not provided by SketchUp in native, even in 2019!
The most popular plugin is TIG Weld. You have also some others: one by Eneroth3 and also Curvizard.