This is not about "rubber stamp effect" this is about creating an image in inkscape for engraving as rubber stamp with lasercut 5.3

So far, I create two shapes - a cirlce inside a square - using inkscape default shapes. I do no fill on both shapes and save the image as dxf via the built dxf option in inkscape.

I import the dxf into lasercut and when I try and simulate the engraving I get "polylines must be closed".

Any help on creating an image in inkscape that can burned by a laser for a rubber stamp will be appreciated.

Welcome to InkscapeForum!

I'm not familiar with your machine, so I'm not sure what it's calling a "polyline". Since you said you used default shapes, it sounds like you must have used the Rectangle and Ellipse tools, for the circle inside a square. Those would create automatically closed paths....assuming that they were converted to paths. But sometimes programs used with Inkscape either can't recognize the real problem, or doesn't have an error message that fits the problem. After you made the circle and square, did you use Path menu > Object to Path? If you didn't do that, the lines wouldn't be paths. And it's my understanding that this kind of machine requires paths.

If you did, did you happen to break the paths for any reason? Or did you maybe use the Pen or Pencil tools to draw the lines?? If it's either one of those might fit, it's possible that the paths are not closed.

I did happen to see a tutorial the other day, that called the Calligraphy tool a polyline tool. Inkscape's Calligraphy tool makes closed paths. So maybe the error message really does mean that the paths are not closed. Or maybe the lines aren't paths, but your machine doens't have an error message that fits, so it throws up the polyline error.

I can think of a couple other "what ifs" so if none of that solves the problem, would you please show us the SVG file?

Try using the path operation [Path>object to path] to convert the primitive polyline shapes to path objects.
Your machine probably needs a path to follow, but svg circles and other primitive shapes are not path objects.
( if your trying to engrave using unconverted shapes, This would explain the Polyline error message You've been getting.)
Best of luck, and please let us know if you succeed.