With NULL don't use '= NULL' or '!= NULL', but use 'IS NULL' or 'IS NOT NULL'. Check item_1 is not null.
Comparisons involving nulls always yield NULL , OR returns TRUE only if either one of the operands is TRUE, AND returns TRUE only if both operands are true.

CREATE TABLE Constraint_Demo
(Item_1 VARCHAR2(10) NOT NULL,
Item_2 NUMBER(10),
CONSTRAINT Tuple_Check CHECK ((Item_1 = 'A' AND Item_2 IS NOT Null) OR (Item_1 = 'B' AND Item_2 IS Null))
);