Probably your problem is that for Docker that has been installed from default Ubuntu repository, the package name is docker.io
Or package name may be something like docker-ce.
Try running
dpkg -l | grep -i docker
to identify what installed package you have
So you need to change package name in commands from https://stackoverflow.com/a/31313851/2340159 to match package name. For example, for docker.io it would be:
sudo apt-get purge -y docker.io
sudo apt-get autoremove -y --purge docker.io
sudo apt-get autoclean
It adds:
The above commands will not remove images, containers, volumes, or user created configuration files on your host. If you wish to delete all images, containers, and volumes run the following command:
sudo rm -rf /var/lib/docker
Remove docker from apparmor.d:
sudo rm /etc/apparmor.d/docker
Remove docker group:
sudo groupdel docker