Why the sync error occurs
This error occurs when a non-USD vendor in Bill.com syncs to QuickBooks Online, but multi-currency isn't enabled QuickBooks Online, so the vendor and their bills are created in USD. Then later multi-currency is enabled in QuickBooks Online, so the vendor's bills in Bill.com now try to sync in then non-USD currency. Vendors can only have one currency in QuickBooks Online.
How to fix the sync error
Create a new vendor in QuickBooks Online or Bill.com with the non-USD currency and sync. The new vendor will then be in both Bill.com and QuickBooks Online, and should be used for future transactions.