I'm trying to mirror a 3d cut extrude (used a 3d sketch as the base), but whenever I do it gives me "need to select a linear edge, sketch line, axis, plane, or planar face to define a directional vector". This is the same error I'd get if I had attempted to just do the 3d cut myself, so I don't know why it's giving me this in a mirror.

Is there a way to give it the vector it wants with the mirror?

Thanks in advance folks, you've been a lot of help on this project.