This error is commonly found in the systems of users who happened to install the latest flavor of UBUNTU from their USB. The error looks similar to this if you happen to run the command
sudo apt-get update from the terminal.
To fix this error, simply launch the ‘ software updater application, click on settings and move to the Other Software tab. Uncheck cdrom: [Ubuntu 16.04.1 LTS_Xenial_Xerus…]
Close ‘ software updater ‘ and tick on ‘ Reload ‘ and you can consider your problem fixed.
To verify that the xenial_xerus error is indeed gone, fire up the terminal and enter
sudo apt-get update and you should be met with no errors.