Sorted issue by downloading “update-notifier” 0.121ubuntu1 source package from https://launchpad.net/ubuntu/+source/update-notifier/0.121ubuntu1, and using "sudo dpkg - i" to install.
Sorted issue by downloading “update-notifier” 0.121ubuntu1 source package from https:/ /launchpad. net/ubuntu/ +source/ update- notifier/ 0.121ubuntu1, and using "sudo dpkg - i" to install.