Try this
CREATE TABLE Product(Id INT, Stock INT,UnitsStock INT)
INSERT Product VALUES(1,100,200),(2,200,400),(3,300,600),(4,400,800)
GO
CREATE TRIGGER trUnitStocks ON Product
FOR UPDATE
AS
BEGIN
DECLARE @UnitStocks INT
SET @UnitStocks = (SELECT I.UnitsStock FROM INSERTED I)
IF @UnitStocks = 0
PRINT 'When unitstocks 0'
--Your code--
ELSE
PRINT 'When unitstocks is not 0'
--Your code--
END
GO
UPDATE Product SET UnitsStock = 100 WHERE Id = 2
Result:-
When unitstocks is not 0
(1 row(s) affected)