Comment 3 for bug 1482838

Looking at the log file for your installation it appears that the disk, likely a USB stick, was not erased before putting 14.04 on it. As we can see here there is information about vivid still on it, which would surely cause the installation failure.

Aug 8 07:17:37 ubuntu apt-setup: Source list entries for this disc are:
Aug 8 07:17:37 ubuntu apt-setup: deb cdrom:[Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805)]/ trusty main restricted
Aug 8 07:17:37 ubuntu apt-setup: deb cdrom:[Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805)]/ vivid main restricted
Aug 8 07:17:37 ubuntu apt-setup: Repeat this process for the rest of the CDs in your set.
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) trusty InRelease
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) vivid InRelease
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) trusty/main Translation-en_US
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) trusty/main Translation-en
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) trusty/restricted Translation-en_US
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) trusty/restricted Translation-en
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) vivid/main Translation-en_US
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) vivid/main Translation-en
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) vivid/restricted Translation-en_US
Aug 8 07:17:38 ubuntu in-target: Ign cdrom://Ubuntu 14.04.3 LTS _Trusty Tahr_ - Beta amd64 (20150805) vivid/restricted Translation-en