ORA-12516, TNS:listener could not find available handler

You opened a lot of connections and that's the issue. I think in your code, you did not close the opened connection.

A database bounce could temporarily solve, but will re-appear when you do consecutive execution. Also, it should be verified the number of concurrent connections to the database. If maximum DB processes parameter has been reached this is a common symptom.

Courtesy of this thread: https://community.oracle.com/thread/362226?tstart=-1

I fixed this problem with sql command line:

connect system/<password>
alter system set processes=300 scope=spfile;
alter system set sessions=300 scope=spfile;

Restart database.