hmm, i have no ubuntu to check whether the install still does what it used to do.
Make sure,
- that apt-get did install the package aolserver4-nspostgres
- that postgres is up and running when you start openacs (postmaster should be listening on port 5432 in the default configuration)
i remember reports, that there was a timing problem for first debian/ubuntu installs, where openacs was starting before postgres was up and running. See e.g. end of thread https://openacs.org/forums/message-view?message_id=138993
