Hi @Mike!
Are you trying to add a new item?
In that case, try by sending a different partner_line_item_id because that one already exists on the order.
Try by not sending the partner_line_item_id or using a different value, because that one already exists.
If you need to update the quantity for an existing SKU on the order try using order_update_line_items
Let me know if that doesn’t help or I could provide a more concrete example.
Thanks again!
Tom