SQL Fiddle Output Error

You need to emulate dbms_output.put_line :)

Schema:

create table Employee(
  name varchar2(100),
  id integer, 
  salary integer,
  PRIMARY KEY(id)
);

insert into Employee(name,id,salary) values('sa',94,100);
insert into Employee(name,id,salary) values('pr',88,150);
insert into Employee(name,id,salary) values('ji',33,900);
insert into Employee(name,id,salary) values('na',24,880);
insert into Employee(name,id,salary) values('po',65,770);
insert into Employee(name,id,salary) values('ri',69,910);
insert into Employee(name,id,salary) values('uj',12,650);
insert into Employee(name,id,salary) values('ad',43,440);
insert into Employee(name,id,salary) values('sam',40,550);

create table dbmsoutput (
  pos int,
  mes varchar2(4000)
);

SQL:

DECLARE
  employee_record Employee%ROWTYPE;
  procedure put_line(p_mes in varchar2) is
     v_pos int; 
  begin  
     select count(0) into v_pos from dbmsoutput;  
     insert into dbmsoutput (pos, mes) values (v_pos, p_mes);
  end;
BEGIN
 put_line('Hello!  This code is powered by dbms_output emulator :)');
 -- Your code here:
 select * into employee_record from Employee where id>90;
 put_line(employee_record.name||' '||employee_record.id||' '||employee_record.salary);
 --
 put_line('Bye!');
END;
/


SELECT mes FROM dbmsoutput order by pos

fiddle

Just as a curiosity really, you can get limited dbms_output results from SQL Fiddle, but you need a function to extract the buffered lines and return them in a form you can select. This uses a pipelined table:

create type t_lines as table of varchar2(4000)
/

create or replace function get_lines
return t_lines pipelined is
  lines dbms_output.chararr;
  numlines integer;
begin
  numlines := 999;
  dbms_output.get_lines(lines, numlines);
  if numlines > 0 then
    for i in 1..numlines loop
      pipe row (lines(i));
    end loop;
  else
    pipe row ('No data');
  end if;
end;
/

And then, after whatever you have issuing dbms_output.put_line calls:

select * from table(get_lines);

Demo. And see the dbms_output documentation to see what its get_lines procedure does and how it relates to your put_lines calls.

But just because you can do something, doesn't mean you necessarily should. This is awkward and doesn't scale, but neither does trying to learn PL/SQL through SQL Fiddle really.

I'd second Ben's recommendation to get your own database to play with, but I'd suggest you look at a pre-built VM image you can run in VirtualBox, which saves you a lot of time in the setup - you don't have to worry about how to install the Oracle software or create and configure a database, it's just ready to use, and you can throw it away or easily start again if things go wrong.