Error upgrading server from 14.04 to 16.04

[Mark Goodrich]

SystemError: E:Sub-process /usr/bin/dpkg returned an error code (2)

Could not install the upgrades

The upgrade has aborted. Your system could be in an unusable state. A
recovery will run now (dpkg --configure -a).

Please report this bug in a browser at
http://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+filebug
and attach the files in /var/log/dist-upgrade/ to the bug report.
installArchives() failed

dpkg: error: dpkg status database is locked by another process

Upgrade complete

The upgrade has completed but there were errors during the upgrade
process.