SQL Server INSERT INTO with WHERE clause

It sounds like having the customerID already set. In that case you should use an update statement to update a row. Insert statements will add a completely new row which can not contain a value.

I think you are trying to do an update statement (set amount = 12.33 for customer with ID = 145300)

UPDATE Payments
SET Amount = 12.33
WHERE CustomerID = '145300'

Else if you are trying to insert a new row then you have to use

IF NOT EXISTS(SELECT 1 FROM Payments WHERE CustomerID = '145300')
    INSERT INTO Payments(CustomerID,Amount)
    VALUES('145300',12.33)

Or if you want to combine both command (if customer exists do update else insert new row)

IF NOT EXISTS(SELECT 1 FROM Payments WHERE CustomerID = '145300')
    INSERT INTO Payments(CustomerID,Amount)
    VALUES('145300',12.33)
ELSE
    UPDATE Payments
    SET Amount = 12.33
    WHERE CustomerID = '145300'

If you want to insert new rows with the given CustomerID

INSERT
    INTO
        Payments(Amount,CustomerID )
VALUES(12.33,'145300');

else if you already have payment for the customer you can do:

UPDATE
        Payments
SET Amount = 12.33
WHERE
    CustomerID = '145300';