causes: A connection to a database or other service was requested using a connect identifier, and the connect identifier specified could not be resolved into a connect descriptor using one of the naming methods configured. For example, if the type of connect identifier used was a net service name then the net service name could not be found in a naming method repository, or the repository could not be located or reached.
Action:
1. Verify the listners are up or not ?
2. verify the tnsnames. ora file location,Make sure the host, port and service name specified are correct.
3. Verify that the tnsnames.ora file has the alias and the service name that the customer is using.
4.Check if the environment variable 'PATH' has the path for tnsnames.ora file specified.