Cannot save Fulfillment => undefined method 'each' for nil

[IP1999]

In the

fulfillment.line_items_by_fulfillment_order 

You need to add

“fulfillment_order_line_items” => [
{
“id” => ,
“quantity” =>
}

https://shopify.dev/api/admin-rest/2022-10/resources/fulfillment#post-fulfillments