Well, it’s good to get the obvious stuff out of the way first.
Yes, a reboot would solve the zombie process thing.
I take it you have checked the device is actually available on USB? On Linux I would use the ‘lsusb’ command, I think on MacOS you’d have to use the System Profiler tool.
If you can’t see it you are back to cables and connectors (or a dead ST-Link)
If that works, next step is try a tool that doesn’t use OpenOCD - eg: the STMCubeProgrammer tool which will try and discover the device and, very usefully, upgrade the software in the ST-Link interface itself (there’s a little STM32 that does the USB to ST-Link task separate to the STM32H743ZI on your Nucleo board.