Hi
As far as I can tell you are using the correct polycurve (Polycurve.ByJoinedCurves) and normal vector nodes. The issue with Curve.Join is that the polycurve it generates is a list of curves and the node will then create separate rebar sections instead of a continuous bar. I’ve found that once you get an internal error on that node, you sometimes need to restart Revit before it will work again, even if you have fixed the input nodes. You don’t need the Group Curves node, you can feed the Flatten node straight into the PolyCurve.ByJoinedCurves node, I don’t think this will affect your error though.
I can’t make out the detail of the polycurves on your image (the image below shows what I think you are trying to make, correct me if I’m wrong), but your bend radii are possibly too small. If you don’t have any radii on the bends and they are just two straight lines meeting at 90 degrees (top in image), the node won’t be able to make the bars (however, you’ll get a different error, and not the internal error warning). Also, if you do have radii at the 90 degree bends, but they are smaller than the standard bend radius, the node also won’t be able to make the bars (but again, this will give you a different error).
Remember that the standard bend diameters are inside diameters and that you are drawing the bar center lines, so you have to adjust your radii accordingly (eg, if you have a 25mm bar, with a standard bend diameter of 155mm, the arcs in your polycurve have to have a diameter of at least 180mm, or the rebar creation node will fail).
But as mentioned above, incorrect geometries would cause different errors, your code looks fine, can’t see why you get the internal error.
If you are trying to create bars with the shape shown in the image, you could just draw the bottom straight section and add 90degree hooks to the ends using the rebar node.