How to mirror copy feature without the "Part not modified: edit the feature inputs" error?

so i try to mirror a perfectly working sketch or a cutout of the same exact sketch to a parallel or coincident plane, but it gives this annoying and random error again ?

lol… now i want to add some more features to a sketch, but why does it delete all of the new features from the side that it should be copied from, not the empty side copied to ?? O__O

Definitely need pics or video to understand what you are referring to.