I had a problem where I had disabled gatekeeper and everything, but PKhex wouldn't open. No error message, it just wouldn't do anything. This worked for me. I'm going to put some common search terms in this post to help people find this thread, because it's clearly the most useful one on the internet. Thank you everyone for contributing.
PKHex won't open mac OS
PKHex won't launch mac OS
PKHex not opening mac OS
PKHex help mac OS