This is a known limitation. AutoTrader Web gets this error whenever it sends a modify order request to Profitmart.
We have approached their support team multiple times, but did not get any useful response from them.
Solution
- Cancel your order & enter a new order with desired modification
- For a permanent fix, raise a request with Profitmart support. We will be more than happy to provide all details of the internal HTTP request & response as well as anything else that is needed from us.