Weird. Adafruit Grand Central debugging with atmel-ice - #15 by spin is using the same chip and those instructions work. He explicitly says
I got the openocd running with Zadig WinUSB driver with arm-none-eabi-gdb and an Atmel sam ice with the Grand Central Express M4.
There also seem to be other USB devices which you may need to reconfigure (or only one of them is correct)
EDBG Data Gateway (Interface 3) driver: winUSB v43.0.0.0
EDBG CMSIS-DAP (interface 0) HidUSB (v10.0.18362.175)
EDBG VIrtual COM Port (interface 1) usbser (v1.1.1.0)
This seems like a pure driver issue here, so all I can recommend is double checking every device driver, replugging the device and trying a standalone openocd command as posted in the linked topic (You should have openocd in C:\Users\<user>\.platformio\packages\tool-openocd which you can use from the commandline)