SQL> create or replace trigger before_in_ddd
  2  before insert on ddd
  3  for each row
  4  declare
  5  a varchar2(10);
  6  begin
  7  if :new.dname='sample' then
  8    raise_application_error(-20001,'enter some other number');
  9  end if;
 10  end;
 11  /

Trigger created.

SQL> insert into ddd values ('samples')
  2  ;

1 row created.

SQL> insert into ddd values ('sample');
insert into ddd values ('sample')
            *
ERROR at line 1:
ORA-20001: enter some other number
ORA-06512: at "MAG.BEFORE_IN_DDD", line 5
ORA-04088: error during execution of trigger 'MAG.BEFORE_IN_DDD'

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u have to use trigger predicates :new and :old.
in your code, u have already insert number 001.
so when u are querying the table first record is 001.
so, the excepption is thrown.
WHICH IS EXACTLY WAT U REQUIRED.
[b] and pls maintain a single thread...people cant kkeep track ....