Looking at the post install script, there might be confusion regarding
- /usr/lib/systemd/system
- /lib/systemd/system
On Ubuntu 20.04 one should be a symlink to the other. However this is not guaranteed after upgrade from older Ubuntu versions. If both directories are different and both directories contain config files with cmk or check-mk in its name: First uninstall, then remove the remaining config files, then reinstall.