query – ON CONFLICT ON CONSTRAINT fail postgresql

INSERT INTO O_USER (ID, NAME, NAME, EMAIL, USER NAME, PASSWORD, DATE_REG, DATE_UPD, DATE_DEL, IS_DELETE, TIPO, FTP)
VALUES (% s,% s,% s,% s,% s,% s,% s, DEFAULT, NULL, NULL,% s,% s,% s)
CONFLICT ON CONSTRAINT DO NOTHING;

ux constraint:

CREATE SINGLE INDEX ux ON public.o_user USING btree (email, username);

but in the multiple insertion have this error:

psycopg2.IntegrityError: The value of the duplicate key violates the unique constraint "o_user_username_key"
DETAIL: The key (username) = (jferraro) already exists.

What's the problem?