Answer
A column's display label in the client doesn't necessarily correspond to an actual API function of a matching name — check the underlying view that the screen is actually built on for the real column name and use that directly in your Data Source, rather than guessing at a hypothetical Get_ function based on the label. If you do need to go through the API rather than the view directly, look for the function that actually exists and check what key/argument it expects (for example, some quantity-related API functions take a specific transaction ID rather than a business-key combination like contract/part number) — it's worth using the API Explorer or the view's own definition to confirm exact names and arguments up front, rather than assuming a Get_ function mirrors the client's column label.