我认为您正在尝试执行更新语句（对于ID = 145300的客户，设置金额= 12.33）

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'

否则，如果要插入新的行，那么您必须使用

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)

您想组合两个命令（如果客户存在，请更新，否则插入新行）

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'