TNS:listener could not find available handler

ORA-12520: TNS:listener could not find available handler

Solution:

1) Check your Listener services for the particular database with this command.
"LSNRCTL Services"

2) It happens that the number of current utilized processes equals the max utilization.

select * from v$resource_limit where resource_name in ('processes','sessions');

Increase the process as in the parameter file or with this alter command & restart the Instance:
ALTER SYSTEM SET PROCESSES=350 SCOPE=SPFILE;