g_dnl_register: failed!, error: -19
The log should check the usb device issue if it matches the customer board hardware.
The bsp for i.MX8MM is enable typeC(CONFIG_USB_TCPC ) in u-boot.
CONFIG_FASTBOOT_USB_DEV is for the usb device index. and don't which BSP version you are using. the old version could not auto detect and need to configure it.
for emmc voltage, if you read the spec, the emmc io 1.8v could work for high speed and legacy speed both. So far, it is not emmc issue.
emmc is not sdcard, sdcard has voltage select when init. and emmc and sdcard belong to different spec.
This is from emmc jedec spec, emmc io could be 1.8v for all speed.
you can double check the hardware mmc io is 1.8v or 3.3v or add no-1-8-v to the device tree(linux\Documentation\devicetree\bindings\mmc\mmc.txt) to try.
and you can boot from sdcard, when you get into linux.
you can check in the /sys fs.
you need to check the usb first if has typeC or not.
