The kernel supplied by Radxa for the Rock3 is too crippled to run Docker.
You will need to follow the instructions to build kernel+Debian, and then wade through the scripts & methods in order to get it all built and installed. It’s a bit of a mess.
You will also have problems with iptables (related to the docker issue), GPT partitions, and (adjusting) zramswap.
It’s quite a process!
OR checkout the OpenVPN thread for an updated kernel.
Having a quick look at the config, it looks like it’s more likely to support docker, but hasn’t got auditing, apparmor+selinux and hugetlb enabled, which the various docker and k3s check-config scripts look for.