Message Oracle NNL-00010 - error getting address for server string

ERRORerror getting address for server string
CAUSEs for the failure. The control programs SQLNET.ORA file must exist in the correct system-specific location, and must be readable by the control program. The file must contain a names.preferred_servers entry with valid name server addresses. If the user has changed managed servers with the SET SERVER command, the new server must be running, and must be able to resolve the address or forward the request to another server which can resolve the address. Alternatively, a TNSNAMES.ORA file with valid name server names and addresses must exist in the correct system-specific location.
ACTIONFor further details, turn on tracing and re-execute the failing operation. If the error persists, contact Worldwide Customer Support.