Workable Copy/Mirror

[Lowlyslows]

Btw, you should not sketch upon face : TNP
You should use axis (at the 'top' of your object).

Is this why I can't change anything? I am trying to change a sketch but nothing happens or it breaks the model.

[papyblaise]

I told you above that you can't do it in PartDesign
if you make a mirror, you will have a single welded part, or if you shift you will have 2 separate parts in the same body from where crash
if you make a clone of Body, you will have 2 pieces but when you want to subtract: either you have subtracted the 1st body on its copy = 0
either you have autointersection from where crash
in my example that I join you you can see that even the A is reproduced symmetrically, so this is the solution (in my previous file I had removed the A for a smaller file) but as that did not suit the commentators ....

[drmacro]

You have not understood the concept of Part Design workbench Body.

The Body constitutes a single cumulative solitary solid. The features in the Body (Pad/Pocket/etc.) do not constitute atomic solids. They are operations that accumulate.

If you use the Part Design Feature Mirror, it creates a mirror of the accumulation up to that point and it is added, making a single solid.

Thus, you can only continue to modify the entire solid Body.

The suggestions offered, create a new solid. This allows you to continue to modify the mirrored solid, but, it is not part of the Body from which it was derived.