O Ubuntu está procurando por cdrom em /etc/apt/sources.list. Para corrigir isso:
sudo nano /etc/apt/sources.list
Remova todas as linhas com cdrom. Salve o arquivo e tente novamente.
Meu terminal diz isso quando eu corro sudo apt-get update
Ign:1 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial InRelease
Ign:2 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial Release
Ign:3 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 Packages
Ign:4 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main i386 Packages
Ign:5 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main all Packages
Ign:6 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en_US
Ign:7 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en
Ign:8 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 DEP-11 Metadata
Ign:9 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main DEP-11 64x64 Icons
Ign:10 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 Packages
Ign:11 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted i386 Packages
Ign:12 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted all Packages
Ign:13 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en_US
Ign:14 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en
Ign:15 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 DEP-11 Metadata
Ign:16 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted DEP-11 64x64 Icons
Ign:3 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 Packages
Ign:4 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main i386 Packages
Ign:5 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main all Packages
Ign:6 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en_US
Ign:7 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en
Ign:8 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 DEP-11 Metadata
Ign:9 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main DEP-11 64x64 Icons
Ign:10 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 Packages
Ign:11 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted i386 Packages
Ign:12 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted all Packages
Ign:13 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en_US
Ign:14 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en
Ign:15 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 DEP-11 Metadata
Ign:16 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted DEP-11 64x64 Icons
Ign:3 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 Packages
Ign:4 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main i386 Packages
Ign:5 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main all Packages
Ign:6 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en_US
Ign:7 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en
Ign:8 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 DEP-11 Metadata
Ign:9 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main DEP-11 64x64 Icons
Ign:10 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 Packages
Ign:11 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted i386 Packages
Ign:12 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted all Packages
Ign:13 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en_US
Ign:14 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en
Ign:15 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 DEP-11 Metadata
Ign:16 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted DEP-11 64x64 Icons
Ign:3 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 Packages
Ign:4 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main i386 Packages
Ign:5 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main all Packages
Ign:6 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en_US
Ign:7 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en
Ign:8 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 DEP-11 Metadata
Ign:9 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main DEP-11 64x64 Icons
Ign:10 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 Packages
Ign:11 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted i386 Packages
Ign:12 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted all Packages
Ign:13 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en_US
Ign:14 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en
Ign:15 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 DEP-11 Metadata
Ign:16 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted DEP-11 64x64 Icons
Ign:3 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 Packages
Ign:4 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main i386 Packages
Ign:5 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main all Packages
Ign:6 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en_US
Ign:7 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main Translation-en
Ign:8 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 DEP-11 Metadata
Ign:9 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main DEP-11 64x64 Icons
Ign:10 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 Packages
Ign:11 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted i386 Packages
Ign:12 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted all Packages
Ign:13 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en_US
Ign:14 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted Translation-en
Ign:15 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted amd64 DEP-11 Metadata
Ign:16 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/restricted DEP-11 64x64 Icons
Err:3 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main amd64 Packages
Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
Ign:4 cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial/main i386 Packages
Ign:17 http://dl.google.com/linux/chrome/deb stable InRelease
Hit:18 http://dl.google.com/linux/chrome/deb stable Release
Hit:20 http://security.ubuntu.com/ubuntu xenial-security InRelease
Hit:21 http://id.archive.ubuntu.com/ubuntu xenial InRelease
Hit:22 http://archive.canonical.com/ubuntu xenial InRelease
Hit:23 http://id.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:24 http://id.archive.ubuntu.com/ubuntu xenial-backports InRelease
Reading package lists... Done
W: The repository 'cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801) xenial Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.E: Failed to fetch cdrom://Ubuntu 16.04.3 LTS _Xenial Xerus_ - Release amd64 (20170801)/dists/xenial/main/binary-amd64/Packages Please use apt-cdrom to make this CD-ROM recognized by APT. apt-get update cannot be used to add new CD-ROMs
E: Some index files failed to download. They have been ignored, or old ones used instead.
O Ubuntu está procurando por cdrom em /etc/apt/sources.list. Para corrigir isso:
sudo nano /etc/apt/sources.list
Remova todas as linhas com cdrom. Salve o arquivo e tente novamente.