It looks like there are old containers of the Checkmk image running, and the volume is not empty.
Try to find old containers with docker ps -a, stop them if needed with docker stop, and remove them with docker rm.
Then, search for the volume with docker volume ls. If there is a line like "local monitoring, remove it with docker image rm monitoring`. After that, the unmodified start command from the Checkmk documentation should work. Do not omit the “uid=1000,gid=1000” options in the tmpfs directive.
If this does not work, install a fresh Debian 11 VM and try again.