Why the Sync Error occurs
This error occurs when a bill is trying to sync to QuickBooks Online (QBO) from Bill.com, and the vendor has different currencies assigned in QBO and Bill.com. Vendor currencies must match between QBO and Bill.com if multi-currency is enabled in QBO.
How to Fix the Sync Error
Vendor currency cannot be changed in Bill.com once a bill is created for the vendor. Vendor currency cannot be changed in QBO either, once saved. To solve this error, a new vendor will need to be created in either QBO or Bill.com with the desired currency.