Explanation
The best thing for the customer service representative to do in this situation is to wait until the Order Summary is created. An Order Summary is a record that represents the financial summary of an order that is received from an external system, such as B2C Commerce or B2B Commerce. An Order Summary is created after an order is ingested into Order Management, and it triggers various flows and processes for order fulfillment and payment processing. A Pending Order Summary is a temporary record that indicates that an order has been received but not yet processed by Order Management. A customer service representative cannot service an order until it has an Order Summary record. Verified References:https://help.salesforce.com/s/articleView?id=sf.order_management_order_summary.htm&type=5

Explanation
Three steps that the administrator should take to implement this functionality are:
Create an External Data Source pointing to the external system's database. An External Data Source is a type of metadata that defines the connection and authentication information for an external system that stores data outside of Salesforce. The administrator can create an External Data Source using the Lightning Connect OData 2.0 or 4.0 adapters, which support read-only access to data in relational databases.
Create an External Object for each table of accounting data in the external system. An External Object is a type of custom object that maps to a table in an external system. The administrator can create an External Object for each table of accounting data that they want to access from Salesforce, and define its fields and relationships accordingly.
Create an Indirect Lookup field to connect to the Account. An Indirect Lookup field is a type of custom field that links a child External Object record to a parent standard or custom object record. The administrator can create an Indirect Lookup field on the External Object that references the Account object, and specify the external column name and value that match the Account ID.
Verified References: https://help.salesforce.com/s/articleView?id=sf.external_object_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.external_object_relationships.htm&type=5

Based on some changes in Universal Containers' Commerce engine, new Orders are expected to have two additional attributes at the header level. In Order Management, besides Order, the other object that also needs to be extended is Order Summary. An Order Summary is a record that stores information about an order, such as the total amount, the number of items, and the fulfillment status. An Order Summary is created by a process called Create Order Summary Process, which runs whenever an order is created or updated. The process calls a flow called Create OrderSummary Flow, which calculates the order summary fields and creates or updates the order summary record. To extend an object in Order Management, an administrator can create custom attributes on that object that match the API name and data type of the corresponding attributes in B2C Commerce. Reference: Order Management Objects

Two reasons that could cause the issue of not being able to deploy a Change Set from a development org to a test org are:
The development org has not been approved to upload Change Sets from within the test org. To deploy Change Sets between two sandbox orgs, the administrator needs to establish a deployment connection between them. A deployment connection allows one org to send outbound Change Sets and another org to receive inbound Change Sets. The administrator can create a deployment connection request from the development org and approve it from the test org.
The development org and test org are not associated with the same production org. To deploy Change Sets between two sandbox orgs, they must be linked to the same production org. A production org is an org that contains live data and business processes. A sandbox org is a copy of a production org that is used for development, testing, or training purposes. Sandbox orgs inherit the deployment connections of their source production orgs. Reference: Deploy Using Change Sets, Deploy a Change Set

Three things that the administrator can do to get more details for debugging when encountering an unhandled fault error are:
Create a fault connector. A fault connector is a special type of connector that handles errors that occur in a flow element, such as an assignment, a loop, or an action. A fault connector can route the flow to another element or end the flow with an error message.
Add a Display Text component which includes {! SFIow.FaultMessage}. A Display Text component is a screen component that displays text on a screen element in a flow. The {! SFIow.FaultMessage} is a system variable that contains information about the error that occurred in the flow, such as the element name, error type, and error message.
Create an error connector. An error connector is a special type of connector that handles errors that occur in a screen element, such as invalid user input or required fields being left blank. An error connector can route the flow to another screen element or end the flow with an error message.
Verified Reference: https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_connector_fault.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_screen_component_display_text.htm&type=5 https://help.salesforce.com/s/articleView?id=sf.flow_ref_elements_connector_error.htm&type=5