I can't install Docker because containerd.io has no installation candidate

It's not the cleanest way to do it I guess, but I had the same problem as you and I changed:

$ sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   $(lsb_release -cs) \
   stable"

to

$ sudo add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
   bionic \
   stable"

because

$(lsb_release -cs)

returns "eoan" (the name of the latest ubuntu version, currently 19.10) and it seems to cause problems. Replacing this with "bionic" (ubuntu 18.10) worked perfectly for me, and I still have no problems 4 months later.

Don't forget to open "/etc/apt/sources.list" and comment-out/delete the line:

deb-src [arch=amd64] https://download.docker.com/linux/ubuntu eoan stable

to avoid conflicts. (You added this with the first command you entered in this process.)

Edit the file : /etc/apt/sources.list

Look for repo entry :

deb-src [arch=amd64] https://download.docker.com/linux/ubuntu eoan stable

Changing it to:

deb-src [arch=amd64] https://download.docker.com/linux/ubuntu bionic stable 

resolved the issue for me