Hi, I have had the same issue with Ubuntu 21.10 (Impish Indri), because it is very new.
I used this guide, but it failed when trying to run sudo apt-get update.
I did the change suggested by salem.aziel .
In /etc/apt/sources.list.d/docker.list I changed impish to hirsute to install the version for Ubuntu 21.04 (Hirsute Hippo).
Installation did run successfully now. Thanks!
Regards,
Florian