Building of Debian buster arm64 image not possible for ROCK Pi N10?

I needed to modify kernel configuration for my ROCK Pi N10. I used the procedure described in .
However, even though the image that I have downloaded from is “Debian buster” for arm64, I can’t rebuild it.

When I repeat the procedure described in “Make rootfs image” section of, I have only the following scripts in rootfs:

the rootfs/ubuntu-build-service directory contains only the following subdirectories:
buster-desktop-armhf packages scripts stretch-base-arm64 stretch-base-armhf stretch-desktop-arm64 stretch-desktop-armhf

So it seems that the provided environment does not enable building 64-bit version of Debain buster.

TIA & Regards,