The RockPro64 has a built in BOOTROM, which checks for valid boot sectors on different devices. In the case of RockPro64 the BOOTROM checks the following:
- Uboot on SPI flash. If found continue booting, if not found go to step 2.
- Uboot on eMMC. If found continue booting, if not found go to step 3.
- Uboot on SD card. If found continue, if not found go to step 4.
- Boot systems on Ethernet. If not found stop booting.
So to be able to boot Manjaro from NVME, you need to have a Uboot that supports NVME (the Manjaro one does) on either SPI, eMMC or SD card. I have never used the SPI, so I will not count that.
You can flash our image to the NVME drive using DD, Etcher or our Manjaro ARM Flasher. But you also need to flash the 2 uboot binaries to the eMMC drive (idbloader.img and u-boot.itb), like described in the uboot flashing instructions here: uboot-rockpro64.install · master · manjaro-arm / packages / core / uboot-rockpro64 · GitLab.
After you do that, make sure that the PARTUUID’s are correct in /etc/fstab and /boot/extlinux/extlinux.conf and it should boot to your NVME drive.