Why do I get a 12154 error?

This error message occurs when a user attempts to logon to an Oracle database. The tnsnames.ora file was not found or has a syntax error. The Oracle communications software (SQL*Net) did not recognize the database connect descriptor as being valid. For GUI utilities, this is the name entered in the third field of the ORACLE logon box.

You can find more information about this error in the Oracle Network Products Messages Manual:

ORA-12154 TNS:Could not resolve service name
 Cause:
   The service name specified is not defined in the TNSNAMES.ORA file.
 Action:
   Make the following checks and correct the error:
   - Verify that a TNSNAMES.ORA file exists and is in the proper
     place and accessible. See the operating system specific manual
     for details on the required name and location.
   - Check to see that the service name exists in one of the
     TNSNAMES.ORA files and add it if necessary.
   - Make sure there are no syntax errors anywhere in the file.
     Particularly look for unmatched parentheses or stray characters.
     Any error in a TNSNAMES.ORA file makes it unusable. See
     Chapter 4 in the SQL*Net Administrator's Guide. If possible
     regenerate the configuration files using the Oracle Network
     Manager.