I am using SW2018. I am having trouble making a mirror of a table leg I have in an assembly. I have the leg and the plain in the assembly as shown in the picture. The mirror dialog box is off to the left on my computer screen, but not shown in the picture. My problem is I can neither click on the leg or the plain and make it show in the mirror dialog box. in other words, I can't create a mirror image of the table leg. I also want to make a Dissolving Mirror Component Feature so it is independent from the original leg, which means I can move it around, twist it and mate it into its proper location in the table. I tried looking at other mirror questions in SW that might answer this question, but I didn't have any luck.
Thanks for any help.