Why the sync error occurs
This sync error occurs when an invoice is syncing from Bill.com to QuickBooks Online, but the invoice number is already used by another Customer.
In Quickbooks Online, duplicate invoice numbers cannot exist for any customer.
How to fix the sync error
To resolve this error, in Quickbooks Online, disable the setting "Custom transaction numbers"
In Quickbooks Online:
- Select the QuickBooks Gear icon
- Under Your Company, select Account and Settings
- Select the Sales tab
- Select the Sales form content section.
- Select the Custom transaction numbers box.
- Select Save
- Select Done
- Sync again