The instructions for Ubuntu/Debian instalation on https://cri-o.io/ don't work any more since apparently the redirections that repository provided in the past are not effective any more. Instructions say that when environment variable VERSION is set to something like:

VERSION=1.21:1.21.3

Then the following command would add an apt repository to the OS:

echo "deb http://download.opensuse.org/repositories/devel:/kubic:/libcontainers:/stable:/cri-o:/$VERSION/$OS/ /" > /etc/apt/sources.list.d/devel:kubic:libcontainers:stable:cri-o:$VERSION.list

The 1st occurrence of $VERSION in above command should instead of 1.21:1.21.3 expand to 1.21:/1.21.3 to produce the correct repository URL.