Thank you for the model sent by email.
In order to get the expected result:
- select the channel (= the supporting beam of the railing) + the 3 joint cuboids (for the "Shear plate" joints) + the railing cuboid.
- then use the Advance Steel “Copy and mirror, 2D” command, and select the 2 points giving the mirror line.
Among all elements copied by mirror, you get the channel copied and mirrored (I think this is what is wanted), but also the kickrail (this is what is wanted).
PS / In case the kickrail is not on the expected side of a railing, you can modify it within the Railing properties dialog, by going to Handrail > Kickrail and then choose e.g. “Right” for the “Alignment” option.