$ sudo apt update
[sudo] password for chetan:
Ign:1 cdrom://Ubuntu 16.04.1 LTS _Xenial Xerus_ - Release amd64 (20160719) xenial InRelease
Err:2 cdrom://Ubuntu 16.04.1 LTS _Xenial Xerus_ - Release amd64 (20160719) 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://us.archive.ubuntu.com/ubuntu xenial InRelease
Hit:4 http://archive.canonical.com/ubuntu xenial InRelease
Get:5 http://us.archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
Hit:6 http://ppa.launchpad.net/kritalime/ppa/ubuntu xenial InRelease
Get:7 http://us.archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
Get:8 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 DEP-11 Metadata [303 kB]
Get:9 http://us.archive.ubuntu.com/ubuntu xenial-updates/main DEP-11 64x64 Icons [190 kB]
Get:10 http://us.archive.ubuntu.com/ubuntu xenial-updates/universe amd64 DEP-11 Metadata [121 kB]
Get:11 http://us.archive.ubuntu.com/ubuntu xenial-updates/universe DEP-11 64x64 Icons [145 kB]
Get:12 http://us.archive.ubuntu.com/ubuntu xenial-updates/multiverse amd64 DEP-11 Metadata [2,516 B]
Get:13 http://us.archive.ubuntu.com/ubuntu xenial-backports/main amd64 DEP-11 Metadata [208 B]
Get:14 http://us.archive.ubuntu.com/ubuntu xenial-backports/universe amd64 DEP-11 Metadata [212 B]
Get:15 http://us.archive.ubuntu.com/ubuntu xenial-backports/multiverse amd64 DEP-11 Metadata [216 B]
Get:16 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Get:17 http://security.ubuntu.com/ubuntu xenial-security/main amd64 DEP-11 Metadata [68.2 kB]
Get:18 http://security.ubuntu.com/ubuntu xenial-security/main DEP-11 64x64 Icons [43.1 kB]
Get:19 http://security.ubuntu.com/ubuntu xenial-security/universe amd64 DEP-11 Metadata [19.4 kB]
Get:20 http://security.ubuntu.com/ubuntu xenial-security/universe DEP-11 64x64 Icons [25.6 kB]
Get:21 http://security.ubuntu.com/ubuntu xenial-security/multiverse amd64 DEP-11 Metadata [212 B]
Reading package lists... Done
W: Target Sources (main/source/Sources) is configured multiple times in /etc/apt/sources.list.d/kritalime-ubuntu-ppa-xenial.list:2 and /etc/apt/sources.list.d/kritalime-ubuntu-ppa-xenial.list:3
E: The repository 'cdrom://Ubuntu 16.04.1 LTS _Xenial Xerus_ - Release amd64 (20160719) 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.
W: Target Sources (main/source/Sources) is configured multiple times in /etc/apt/sources.list.d/kritalime-ubuntu-ppa-xenial.list:2 and /etc/apt/sources.list.d/kritalime-ubuntu-ppa-xenial.list:3