Hi Wayne, the command I used was
sudo ./tools/kernel_flash/l4t_initrd_flash.sh --external-device nvme0n1p1 -c tools/kernel_flash/flash_l4t_external.xml -p "-c bootloader/t186ref/cfg/flash_t234_qspi.xml" --showlogs --network usb0 p3509-a02+p3767-0000 internal
The NVME had 5 partitions (one FAT and the rest EXT4) on it and this was causing the flash to fail on my side. Once I formatted it and created one EXT4 partition, flashing completed successfully.