@Abhay Can you try one more method
Downloaded both boards, “Seeed nRF52 Boards” and “Seeed nRF52 mbed-enabled Boards.”
Copy the adafruit-nrfutil from
/Users/<user>/Library/Arduino15/packages/Seeeduino/hardware/mbed/2.9.2/tools/adafruit-nrfutil/macos/adafruit-nrfutil
to
/Users/<user>/Library/Arduino15/packages/Seeeduino/hardware/nrf52/1.1.8/tools/adafruit-nrfutil/macos/adafruit-nrfutil
Then chmod -x the copied file
Please check and let me know. Src