Tall Birch

…but I know the database is running, tnsnames.ora is correct and tnsping to that connection is returning ‘OK’. Using sqlplus to connect to the database is returning the error

ORA-12545: Connect failed because target host or object does not exist

what is it?

check to see if your environment variable ‘TWO_TASK’ is set. Once you set this variable to the connection descriptor in your tnsnames.ora file, you’ll likely be ok.