Error when deleting an Oracle table reference

Question

I am trying to delete a record from my table TB_CONTATO who is the daughter of the table TB_CLIENTE:

DELETE FROM TB_CONTATO  WHERE ID_CLIENTE = 2999

But it occurs of violation of integrity:

ORA-02292: integrity constraint (SISMET.SYS_C00418600) violated - child record found <br>

This is the daughter table structure (TB_CONTATO):

CREATE TABLE tb_contato (
  id_contato               NUMBER(10,0) NOT NULL,
  id_cliente               NUMBER(10,0) NOT NULL,
  st_nome                  VARCHAR2(50) NOT NULL
)
  STORAGE (
    NEXT       1024 K
  )
/
PROMPT ALTER TABLE tb_contato ADD CONSTRAINT pk_contato PRIMARY KEY
ALTER TABLE tb_contato
  ADD CONSTRAINT pk_contato PRIMARY KEY (
    id_contato
  )
  USING INDEX
    STORAGE (
      NEXT       1024 K
    )
/

PROMPT ALTER TABLE tb_contato ADD FOREIGN KEY
ALTER TABLE tb_contato
  ADD FOREIGN KEY (
    id_cliente
  ) REFERENCES tb_cliente (
    id_cliente
  )
/
GRANT DELETE,INSERT,SELECT,UPDATE ON tb_contato TO sismetop;

I executed the command line according to this example (Oracle Alter foreign table discard key): ALTER TABLE TB_CONTATO DROP CONSTRAINT SISMET.SYS_C00418600 and generated the following error:

ORA-01735: invalid ALTER TABLE option

I searched on some websites for example Oracle PRIMARY KEY, by which I understood nicely I would do the following :

ALTER TABLE TB_CONTATO
DROP CONSTRAINT pk_nome_da_constraints;

In the example above, I don’t have the name of constranin, as I do to eliminate this reference, this dependence between the Daughter table and the Pail table ?