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

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.

TNS listener could not find available handler for requested type of server.
This error is related with out of available Oracle processes.
Then Increase the value of the PROCESSES initialization parameter.

Solution:
Step-1
SQL> select current_utilization, limit_value ,resource_name
From v$resource_limit
Where resource_name IN ('sessions','processes'); 

CURRENT_UTILIZATION LIMIT_VALU RESOURCE_NAME
------------------- ---------- ------------------------------
                 199        200 processes
                 203        328 sessions 
Step-2
SQL>alter system set processes=400 scope=spfile;
Step-3
SQL>shut immediate
Step-4
SQL>startup
Step-5
SQL> select current_utilization, limit_value ,resource_name
From v$resource_limit
Where resource_name IN ('sessions','processes'); 

CURRENT_UTILIZATION LIMIT_VALU RESOURCE_NAME
------------------- ---------- ------------------------------
                 23        400 processes
                 23        624 sessions