package postfix (not installed) failed to install/upgrade: subprocess installed post-installation script returned error exit status 75

E: Could not get lock /var/cache/apt/archives/lock - open (11: Resource temporarily unavailable)

E: Unable to lock directory /var/cache/apt/archives/

This is Ubuntu 12.04 LTS

ProblemType: Package
DistroRelease: Ubuntu 12.04
Package: postfix (not installed)
ProcVersionSignature: Ubuntu 3.11.0-19.33~precise1-generic 3.11.10.5
Uname: Linux 3.11.0-19-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.6
AptOrdering: postfix: Configure
Architecture: amd64
Date: Thu Apr 10 13:23:00 2014
DuplicateSignature: package:postfix:(not installed):subprocess installed post-installation script returned error exit status 75
ErrorMessage: subprocess installed post-installation script returned error exit status 75
InstallationMedia: Ubuntu 12.04.4 LTS "Precise Pangolin" - Release amd64 (20140204)
MarkForUpload: True
SourcePackage: postfix
Title: package postfix (not installed) failed to install/upgrade: subprocess installed post-installation script returned error exit status 75
UpgradeStatus: No upgrade log present (probably fresh install)