I am getting the following error message when I try to add an item to an existing internal order, "atcClickedSC internal error: no fulfillment method." What does this mean?
That product is likely designated as a ship only product, and you are adding it to an order for pickup, or vice versa. You'd either need to create a new order for the customer to ship/pickup that item, or edit the product to enable the method of the existing order (shipping/pickup and either select a pickup location or ship from address). In the storefront, it appears you can place a single order for ship and pickup, but in reality, in the back end they are kept separate.