Comment 2 for bug 1899682

Thank you for taking the time to file a bug report.

I tried reproducing the upgrade path here inside a lxd container. Of course, since some of the versions you mentioned above are EOL'ed, I started with bionic and upgraded to focal. Everything worked. I made sure to have postfix, postfix-doc and all the other postfix-* packages installed. I also tried purging/removing some postfix-* packages, and then postfix, and then postfix-doc, but everything worked. Your logs tell me that the prerm script of postfix-doc could not locate the /etc/postfix/main.cf file. This is what the script does:

case "$1" in
    remove|upgrade|deconfigure)
        if [ -x /usr/sbin/postconf ]; then
            postconf -e readme_directory=no html_directory=no
        fi
        ;;
...

The only way I could reproduce the failure you've seen was if I manually removed/renamed /etc/postfix/main.cf, and the "apt remove postfix-doc":

Removing postfix-doc (3.4.13-0ubuntu1) ...
postconf: fatal: open /etc/postfix/main.cf for reading: No such file or directory
dpkg: error processing package postfix-doc (--remove):
 installed postfix-doc package pre-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
 postfix-doc
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)

I could not think of a way to remove postfix and still have /usr/sbin/postconf available. From your logs (especifically the DpkgTerminalLog.txt), it is not clear whether postfix was removed before the other postfix-* packages, but I very much doubt this was the case.

Since it seems likely to me that this is a local configuration problem,
rather than a bug in Ubuntu, I am marking this bug as 'Incomplete'.

However, if you believe that this is really a bug in Ubuntu, then we would
be grateful if you would provide a more complete description of the problem
with steps to reproduce, explain why you believe this is a bug in Ubuntu
rather than a problem specific to your system, and then change the bug
status back to "New".

For local configuration issues, you can find assistance here:
http://www.ubuntu.com/support/community