3d polyline.

A polyline is only a 1D entity, not a 2D entity - It is just a line - it has no width. To create width, you need to create a plane - eg, draw a shape with lines, and finish with pressing "C" for "Close" - This will create a 2D shape - So to create width, create a line that has four pieces as a thin rectangle. It would be possible to write a small script to do this automatically, but it's rare that you make a shape with a line alone - eg, to create a box, you don't create 6 faces with thickness, you create two squares, extrude them, then subtract one from the other.

Anyway, I think this is what you're missing - If you create thickness by building a shape with lines, that is closed off, then you can extrude it, and this can be converted to STL for printing.

Regards
David