On Linux Mint, the official instructions did not work for me. I had to go into /etc/apt/sources.list.d/additional-repositories.list and change serena to xenial to reflect my Ubuntu codename. Depending on your Debian variant, version, and the original installation method followed, you may need to modify /etc/apt/sources.list.d/docker.list instead.
You can typically find the appropriate codename by running one of a few different commands. In the following examples, focal is the codename:
$ grep CODENAME /etc/os-release
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
$ lsb_release -c
Codename: focal
# NOTE: On Ubuntu 20.04.2, /etc/os-release is symlinked to /usr/lib/os-release and
# lsb_release reads from /usr/lib/os-release.