Explanation:

Three possible causes of this issue are:

The administrator has duplicate Product SKUs in the JSON query. A Product SKU is a unique identifier for a product that is used to track inventory and sales. A Product SKU must be unique within an org, and it cannot be duplicated in a composite API call. If the administrator has duplicate Product SKUs in the JSON query, it will cause an error when loading test orders via the Workbench.

The number of subrequests in the JSON query exceeds the 20 subrequest limit. A subrequest is a single HTTP request that is part of a composite API call. A composite API call can contain up to 20 subrequests in a single JSON body. If the administrator has more than 20 subrequests in the JSON query, it will cause an error when loading test orders via the Workbench.

Record IDs used within the request are incorrect. A record ID is a unique identifier for a record that is used to reference and manipulate data in Salesforce. A record ID must be valid and exist in the org, and it must match the data type and format of the corresponding field. If the administrator has incorrect record IDs in the JSON query, such as using 15-character IDs instead of 18-character IDs, or using IDs from a different org, it will cause an error when loading test orders via the Workbench.

Verified References: https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/resources_composite_composite.htm
https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_composite_sobject_tree_flat.htm

Explanation: The design element in Flow that allows the admin to reset some variables at the beginning of each loop iteration is Assignment. An Assignment element lets the admin assign new values to one or more variables or sObject fields in a flow. The admin can use an Assignment element before a Loop element to initialize the variables that are used in the loop, and then use another Assignment element inside the loop to reset the variables for each iteration.

Explanation: The correct time and way for creating an Invoice is at any point after the Fulfillment Order is created, by customizing flow “Create Invoice”. This flow is a core action that creates an Invoice record from a Fulfillment Order record, and updates the status of both records accordingly. The administrator can customize this flow to suit their business needs, such as adding conditions, actions, or subflows.

Explanation: A summary object represents a dynamic view of underlying data. A summary object is a type of object that aggregates data from related records and displays it in a single record. For example, an Order Summary object represents the financial summary of an order that is received from an external system, such as B2C Commerce or B2B Commerce. An Order Summary object aggregates data from related Order Item Summary records and displays it in fields such as total amount,tax amount, discount amount, etc.

Explanation: The object that the Ensure Funds Apex Flow action looks for when resolving Invoice balances associated with an Order Summary is Order Payment Summaries. This object represents the payments made for an order, and it has a lookup relationship to both Invoice and Order Summary objects. The Ensure Funds Apex Flow action uses this object to calculate the balance due for each Invoice and update its status accordingly.