Oracle Errors Interview Question:

ORA-12516: TNS:listener could not find available handler with matching protocol stack?

Submitted by: Administrator

Cause: None of the known and available service handlers for
the given SERVICE_NAME support the client's protocol stack:
transport, session, and presentation protocols.

Action: Check to make sure that the service handlers (e.g.
dispatchers) for the given SERVICE_NAME are registered with
the listener, are accepting connections, and that they are
properly configured to support the desired protocols.
Submitted by: Administrator