Thanks, Maachan,
I did according to your sql statement, and find the constraint name of the parent table and I enabled it. And then run the query in my first post, but it still gave the same error message. Why is that?
The following is what I did:

SQL> SELECT r_constraint_name
2 FROM DBA_CONSTRAINTS
WHERE CONSTRAINT_NAME='FKINLINEAD01';

R_CONSTRAINT_NAME
------------------------------
PKADAGENCY
PKADAGENCY



SQL> alter table adagency enable constraint pkadagency;

Table altered.