I have been trying to use the mirror modfyer.
I do control r to split the cube in midde then delete that side then I go to click on the modifyer to bring up all the options scroll to mirror I click it and nothing
shows up.
Choose a different axis instead of the default X in the modifier settings.
when you cut it, were you in front view? if not, it may not work. i had the same problem a while ago.
It worked for me thank you for your help