why does this cause an error?

CREATE OR REPLACE TRIGGER mm_return_trg

AFTER UPDATE OF checkin_date ON mm_rental /* After: helps define timing of trigger. UPDATE: event causing trigger to fire */

FOR EACH ROW /* helps define timing of trigger */

BEGIN

IF :OLD.checkin_date IS NOT NULL THEN /* states if old date received data field is NOT empty, continue */

UPDATE mm_rental /* updates mm_rental table */

SET checkin_date = SYSDATE;

UPDATE mm_movie /* updates mm_movie table */

SET movie_qty = movie_qty+1;

COMMIT;

END IF;

END;