Just to add to what Dineesh said, when we get this error, we have you run, on the Oracle server
sqlplus /nolog
connect username/password@service as sysdba
'service' may, or may not be the name of the instance. It can be seen in the 'tnsnames.ora' file
This is how Data Protector trie to log into Oracle, so if the command fails the integration is not going to work