set serveroutput on

PL/SQL is a block  always required to server output result is to be  show on  the screen
 otherwise output will not display .
Whenever you will work on sql prompt  type  "set serveroutput on". command.

SQL> set serveroutput on

Set serveroutput on Example.

SQL> set serveroutput on
SQL> DECLARE
 eno number(5) NOT NULL := 2
 ename varchar2(15) := Jay Patel;
 edept CONSTANT varchar2(15) := Web Developer;
BEGIN
 dbms_output.put_line('Declared Value:');
 dbms_output.put_line(' Eno: ' || eno || ' EName: ' || ename);
 dbms_output.put_line('Constant Declared:');
 dbms_output.put_line(' EDept: ' || edept); 
END;