sudo apt update
[sudo] password for satrap:
Ign:1 cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial InRelease
Err:2 cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial Release
  Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
Hit:3 http://in.archive.ubuntu.com/ubuntu xenial InRelease
Hit:4 http://ppa.launchpad.net/webupd8team/java/ubuntu xenial InRelease
Get:5 http://in.archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
Hit:6 http://ppa.launchpad.net/webupd8team/sublime-text-3/ubuntu xenial InRelease
Get:7 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Hit:8 http://archive.ubuntu.com/ubuntu xenial InRelease
Get:9 http://in.archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
Reading package lists... Done
E: The repository 'cdrom://Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1) xenial Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.