If you are getting the order message:
"The order contains items that cannot be produced at a single production location. Please resubmit as separate orders as follows: Order 1 with item reference IDs: x, y, Order 2 with item reference IDs: z"
it means that you are trying to place in the same order items that will be produced in different locations. This is currently not possible with Gelato.
The solution is to create multiple orders, each with a group of product that will be produced in the same location. In the example above, item reference IDs x and y (Order 1) can be produced together, while item reference ID z (Order 2) has to be produced in a different location.
Note: If the orders are placing through Shopify or Etsy, your customers will still be able to place the order but the order will fail after syncing with Gelato. In this case, you will then need to manually split the orders.