ORA-12519: TNS:no appropriate service handler found.

This error is mainly due to the low "PROCESSES" initialization parameter value.
PROCESS - parameter describes the total number of operating system process allowed to access the database concurrently.

This total process is divided into user process and background processes, user processes are the processes allotted to the users connected to the database and the background processes are the processes utilized by the database.

Procedure to increase the PROCESSES parameter
Login as SYS user

 SQL>alter system set processes=200 scope=spfile;  

then stop and start the DB server.

 SQL> shutdown immediate;  
 SQL> startup;