Try booting OS X into safe mode and try again. To get into safe mode, reboot and hold down the shift key until you see the spinning circle. When you get to the login screen, enter your username and password, but before you hit enter, hold down the shift key again and keep holding it while you click the login button.
Also, when running the necflash command, make sure you prefix it with sudo so as to run as root. You’ll need to type in an admin password.