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

Cause:-
The most frequent cause is the smaller value of the parameter "PROCESSES" set in Oracle, which needs to be increased in order to solve this issue

PROCESSES specifies the maximum number of operating system user processes that can simultaneously connect to Oracle

Value of this parameter can be fetched using query:-

SQL> show parameter process

SQL>SELECT name, value FROM gv$parameter WHERE isdefault = 'FALSE'; 

Solution:-
Login as a system DBA (conn sys as sysdba)

SQL> alter system set processes=200 scope=spfile

Restart the oracle database and the newly entered value for processes parameter will come into effect.