Three possible causes of this issue are:

The administrator hasduplicate 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 compositeAPI 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