The documentation does not say that the default config file exists in /home/username/.curator/curator.yml
It says If --config and CONFIG.YML are not provided, Curator will look in ~/.curator/curator.yml for the configuration file.
The file must be created by the end user.
Also, if you installed via:
sudo apt-get update && sudo apt-get install elasticsearch-curator
but did not add the official Elastic repository for Curator, then you installed an older version. Please check which version you are running with:
$ curator --version
curator, version 5.4.1
If you do not see the current version then you do not have the appropriate repository installed.