Beginner having issue with mirroring

I am following along with Furniture Design in Shapr3D Using PARAMETRIC MODELING - CAD Modeling For Woodworkers by Bevelish Creations on youtube and am having issues mirroring the drawer fronts and sides. I selected the two drawer sides and they turn blue. Then I selected the plane and it turns purple. I then get an error message: Operation failed because the resulting body wouldn’t be valid. There is no more information as to why or what is wrong and I have no idea. Just to see what would happen I made the plane blue and the drawer side purple and it mirrored the plane out to the side of the project. Please help.

After looking at the photo I uploaded I noticed I only had the face selected. I tried the whole body and it worked.