Oracle Errors Interview Question:

ORA-12520: TNS:listener could not find available handler for requested type of server?

Submitted by: Administrator

Cause: None of the known and available service handlers for
requested type of server (dedicated or shared) are
appropriate for the client connection.

Action: Run "lsnrctl services" to ensure that the
instance(s) have registered with the listener and that the
appropriate handlers are accepting connections.
Submitted by: Administrator