Oracle-xe on Ubuntu 18.04 - Installation and impdp
Ubuntu is not a supported distribution for running Oracle databases. Just use a supported distribution.


Operating System Requirements


The database should be configured and created with /etc/init.d/oracle-xe configure. If you do not have a working databases after that, that is an environmental issue: use a supported environment.
I had to check and run createdb.sh.  It created an incomplete database that gave the same error. Instead of it use the above method for creating the database. If it does not work: use a supported environment.


At this point you can start over and try to get the official steps working by folowing random blog posts and forum comments like above. Maybe it will work if you do not use createdb.sh. Or you can save a lot of time by using a supported environment.