带有INSERT语句的WHERE子句，WHERE处的“不正确的语法”，以及IF NOT NOT EXISTS运行，无论是否存在记录

I have this INSERT query:

我有这个INSERT查询:

INSERT INTO Appointments (StaffID, CustomerID, TimeSlot, AppDate) 
VALUES (1, 11, 1, DATEADD(day, 1, GETDATE()))
WHERE NOT EXISTS 
    (SELECT * FROM Appointments 
     WHERE StaffID = 1 AND CustomerID = 11 AND TimeSlot = 1 AND AppDate = DATEADD(day, 1, GETDATE()));
INS