The person that said to put "sudo spctl --master-disable" in the terminal, follow those instructions, then shut down your computer and as soon as you turn it back on hit these buttons all at one time "option, command, p, and r" until the system resets one time then release those buttons after 20 seconds and let it restart for a second time. Then you should be fine and try to log in to your system to try the update one more time. I had the gatekeeper policy error and the "%@" error for the update and I did both of these steps according to what an Apple rep told me to do. Good luck!