I’m not on a Mac (on Linux); I’ve seen that a few times.
Mostly it’s because you have an OpenOCD zombie process hanging around (kill process, or reboot machine)
Sometimes it’s because your OS has lost the plot in regard to USB devices (ie: unplug ST-Link, re-plug)
The other day, it was driving me nuts and I realised that my USB hub had crashed (!); unplugged the hub and re-plugged and everything was good again.