Normally, this error will pop up when you attempt to restart a Plesk-related service, like httpsd, psa-spamassassin or qmail:
Error: HTTPD_INCLUDE_D not defined
This basically means that Plesk is unable to get some required configuration directives from the /etc/psa/psa.conf file. If you can’t find the directive in the file that Plesk is complaining about, check your Plesk RPM’s with
# rpm -q psa
Most likely, you will find that there is a psa-7.5.4 RPM and a psa-8.1.0 or psa-8.1.1 RPM installed simultaneously. This generally appears because of a botched upgrade that was started within Plesk by the admin user.
To fix the issue, get the psa-7.5.4 RPM from autoinstall.plesk.com. Remove the psa-8.1.1 RPM and install the psa-7.5.4 RPM again rather forcefully:
# rpm -ev psa-8.1.1...
# rpm -Uvh --force --nodeps psa-7.5.4...
# /etc/init.d/psa restart
At this point, you can download the command line autoinstaller and try the Plesk upgrade again.
Further reading: http://forum.swsoft.com/showthread.php?threadid=32299