Comment 2 for bug 695124

Setting up logtail (1.3.7ubuntu1) ...
Setting up logcheck (1.3.7ubuntu1) ...
Adding user logcheck to group adm
newaliases: fatal: open /etc/postfix/main.cf: No such file or directory
Password:
chfn: PAM authentication failed
dpkg: error processing logcheck (--configure):
 subprocess installed post-installation script returned error exit status 1

The first message:
newaliases: fatal: open /etc/postfix/main.cf: No such file or directory

seems to indicate that you have a broken newaliaes / postfix configuration (newaliases somehow tries to poke etc/postfix/main.cf which means newaliases is provided by postfix, but somehow it's not there). This would make sense is postfix was being installed in the same dpkg run, but it's not, so I suspect your setup might be broken there?

The second set of messages:
Password:
chfn: PAM authentication failed

suggests that you have a broken PAM configuration: the postinst calls chfn to set the real name of the account in the passwd database:
    chfn -f 'logcheck system account' logcheck
and the postinst runs as root; by default, etc/[pam.d/chfn contains:
auth sufficient pam_rootok.so
[...]
@include common-auth
@include common-account
@include common-session

so it should just accept to run chfn and not prompt you for a password; again, I'm puzzled, did you change your PAM configuration in some way?