Scope
Quickbooks sync error: TRANSACTION - CREATE_FAILED (null)
Solution
You will receive an error message that says "NULL" when the dependent entity of the customer record such as plans, addons, or customer details are deleted in Quickbooks which is the reason these invoices are not syncing.
You can either create a new customer record relevant to in Quickbooks and share the QBO-internal ID at support@chargebee.com so that we can perform the backend mapping. For bulk operations, share the customer IDs of these invoices in a CSV file.