To solve this error, you have to check if this /var/lib/docker/aufs/mnt is mounted or not. To check you can use mount -l command.
If the folder is mounted, then follow the bellow given steps.
$ docker stop -f $(docker ps -a -q) //it will stop all the containers.
$ docker rm -f $(docker ps -a -q) //Remove all the containers.
$ docker rmi -f $(docker images -q) //Remove all the images.
Hope this will work.