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'

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.

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';

Do you want to perform update;

update Payments set Amount  = 12.33 where Payments.CustomerID = '145300'

Better solution and without risk of deadlocks:

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

INSERT INTO Payments(CustomerID,Amount)
    SELECT '145300',12.33
WHERE @@ROWCOUNT=0

i do inserts into a table if the record doesn't exist this way. may not be entirely what is after but it may be helpful

insert into x (a,b)
select 1,2
where 0=(select count(*) from x where a = 1 and b = 2)

Ok, looks like I actually need to just do an insert into the Payments table that has the correct CustomerID, as there are currently no Payments with that CustomerID, so I cannot update it.

I ran INSERT INTO Payments (CustomerID, Amount, PaymentTypeID) Values ('145300', 24.99, 8); and then SELECT * FROM Payments WHERE Payments.CustomerID = '145300'; to confirm and we're in business. Thanks everyone!

Try to update as below if you want to modify existing row,

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