CREATE TABLE tab_1
(
  Col_1 VARCHAR2(10)
);

INSERT INTO tab_1 VALUES ('A');
INSERT INTO tab_1 VALUES ('B');

SELECT * from tab_1;

Col_1
----------
A
B

CREATE TABLE tab_2
(
  Col_2 VARCHAR2(10)
);

INSERT INTO tab_2 VALUES ('C');
INSERT INTO tab_2 VALUES ('D');
INSERT INTO tab_2 VALUES ('E');
INSERT INTO tab_2 VALUES ('F');

SELECT * from tab_2;

Col_2
----------
C
D
E
F

UPDATE tab_1
SET col_1 = (SELECT col_2 FROM tab_2)
where col_1 = 'A';

ORA-01427: single-row-subquery returns more than one row

UPDATE tab_1
SET col_1 IN (SELECT col_2 FROM tab_2)
where col_1 = 'A';

ORA-00927: missing equal sign