I wanted to install 5.9 kernel on arm64 from "https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.9/".

Screenshot from 2020-11-30 13-49-45.png

It listed 3 deb packages above. When I install them, I failed to install headers package since it reported missing 5.9.0-0900 package. When I checked the part of amd64, I found there's a one package in amd64. But it's missing in arm64. So I think it's a bug.