I happened to install the Oracle Instant Client (instructions here [https://chronicler.tech/quickest-way-to-set-up-the-oracle-instant-client-for-sqlplus-access-to-the-oracle-database/] ) to quickly connect to a database using sqlplus. Problem After installation, I simply tried to connect to my database as follows: ./sqlplus scott@dbhost:1521/dsoa.revelationtech.com And got the ORA-12504 error shown here: SQL*