Check that you have specified a valid service name with the SID
attribute in odbc.ini.
If the service name is valid, it may be that the database service is not running or has not registered with the listener.
Failure to register with the listener may be a temporary condition. For example, the database may not have registered yet because the listener has only recently been started. It is therefore worth waiting a few moments and then reconnecting to your ODBC data source, before contacting your database administrator.