ORA-12518: TNS:listener could not hand off client connection

The "ORA-12518: TNS:listener could not hand off client connection" error indicates the Oracle database can't accept new connections from clients

To solve this error:

Increase the value of PROCESSES parameter in the SPfile to a larger value and then restart the database

SQL> CONNECT / AS SYSDBA

SQL> ALTER SYSTEM SET PROCESSES=500 SCOPE=SPFILE;

SQL> SHUTDOWN IMMEDIATE

SQL> STARTUP