The reason why this happening is because you dont have an /etc/aliases
Can you do the following:
sudo touch /etc/aliases
sudo chmod 644 /etc/aliases
** Changed in: postfix (Ubuntu)
Status: New => Won't Fix
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https://bugs.launchpad.net/bugs/798874
Title:
package postfix 2.8.1-1~lucid1 failed to install/upgrade: subprocess
installed post-installation script returned error