clariion's post in ZFS on RockPro64 Armbian current image was marked as the answer
After a few hours, I finally made it with current Focal release. Thanks to @jbergler
Linux RockPro64 5.8.17-rockchip64 #20.08.21 SMP PREEMPT Sat Oct 31 08:22:59 CET 2020 aarch64 aarch64 aarch64 GNU/Linux
# sudo add-apt-repository ppa:ubuntu-toolchain-r/ppa
# sudo apt update
# sudo apt upgrade
# sudo apt install g++-10
# armbian-config -> Software -> Headers install
# mkdir zfs-scratch
# cd zfs-scratch
# apt-get download linux-headers-current-rockchip64
# git clone -b zfs-0.8.5 https://github.com/openzfs/zfs.git
# apt install build-essential autoconf automake libtool gawk alien fakeroot dkms libblkid-dev uuid-dev libudev-dev libssl-dev zlib1g-dev libaio-dev libattr1-dev libelf-dev python3 python3-dev python3-setuptools python3-cffi libffi-dev
# dpkg -i linux-headers-current-*.deb
# cd zfs
# sh autogen.sh
# ./configure
# make
# make install
# sudo apt install spl-dkms
# sudo apt install zfsutils-linux
# reboot
