Why the Sync Error occurs
This error occurs when the chart of account on a bill trying to sync to QuickBooks Online being has either been deleted or is no longer linked between QuickBooks Online and Bill.com.
How to Fix the Sync Error
If the account was deleted in QuickBooks Online, restore the account and sync again.
If you don't want to restore the account in QuickBooks Online, change the account on the bill in Bill.com to an account that is active in QuickBooks Online and sync.
If the account is active in QuickBooks Online, or not there at all contact Customer Support to resync or relink the account.