It seems that in this sales order, you need to add the right variant (child item) of one of its products (parent item) because the parent item doesn't have any SKU associated with it.
Example:
"12345" Sales order not created for the customer with NS ID - "____" = [Code=USER_ERROR] Please choose a child matrix item.
In the above example, this "12345" sales order contains 1 product that has variants in your store, however, none of these variants (child item) is selected in this order.