I'm assuming that you are talking about the ORA-12154 error message (TNS could not resolve service name). When you try to connect to a database, you give three pieces of information, the userid, the password, and the service name. The Oracle software will look up the service name in your %ORACLE_HOME%networkadmintnsnames.ora file. You will have to add the entry to this file with the Net Manager GUI tool that was installed when you install Oracle.
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