Did you compile the plug-in with Java 8 or options to generate Java bytecode compatible with Java 8 (or Java 11 or 15 if you use Sweet Home 3D with those versions)?
You could also create the stair runner with a smaller staircase without stringers and handrails. Just elevate it a bit to make it appear on top of the stairs of the main staircase, like in the following example.
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Emmanuel Puybaret, Sweet Home 3D creator
