One of the configuration options in the Customer Import data flow is to set the imported Customers as child records of a default Parent Customer. Unfortunately, NetSuite places a limitation of a maximum of 10,000 subcustomers (all levels included) in a customer hierarchy. Once this limit is reached, NetSuite will not allow the creation of a new subcustomer. This will cause the Order Import data flow to fail.
Reference: NetSuite Help Guide
"NetSuite limits the maximum number of customers you can include in a customer-subcustomer hierarchy. A customer hierarchy is comprised of the top-level parent customer, all of its subcustomers, and all of their subcustomers. You can have a maximum of 10,000 customers and subcustomers in a customer hierarchy.
This limit is in place to improve performance. If you need to increase your account's limit, please contact NetSuite Customer Support. Going beyond the set hierarchy limit may cause a performance impact related to customer credit limits, customer balances and consolidated balances, as well as payments to the top-level customer."
There are two alternate solutions.
The first solution is to configure the Customer Import data flow to add the Customers to a new, or different, Parent Customer record.
Alternately, if the use of a Parent Customer is not needed, this issue can be resolved by removing the Parent Customer mapping. The default connector setup only uses one default Customer.