it worked with following steps thanks Mr. Pando for
participating & help and thanks to denevge too.

SQL> create table aa (code char(2) constraint pk_1 primary key,name char(20) )

Table created.

SQL> insert into aa values('11','anuj1') ;

1 row created.

SQL> commit ;

Commit complete.

SQL> insert into aa values('11','anuj2') ;
insert into aa values('11','anuj2')
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.PK_1) violated


SQL> alter table aa drop constraint pk_1 ;

Table altered.



SQL> insert into aa values('11','anuj2') ;

1 row created.

SQL> commit ;

Commit complete.

SQL> alter table aa add constraint
2 pk_1 primary key (code) deferrable novalidate
3 /

Table altered.

SQL> insert into aa values('11','anuj3) ;
insert into aa values('11','anuj3)
*
ERROR at line 1:
ORA-01756: quoted string not properly terminated