Oracle Message TNS-12520 - TNS: listener could not find available handler for requested type of server

ERRORTNS: listener could not find available handler for requested type of server
CAUSENone of the known and available service handlers for requested type of server (dedicated or shared) are appropriate for the client connection.
ACTIONRun lsnrctl services to ensure that the instance(s) have registered with the listener and that the appropriate handlers are accepting connections.