Why the Sync Error occurs
This error occurs when an account is deleted in Bill.com and the deleted account is attempting to sync to QuickBooks Online (QBO), but the account is one of the special accounts QBO needs to operate uniquely (like __Uncategorized Income __and Uncategorized Expense).
How to Fix the Sync Error
These accounts cannot be deleted or edited in QBO. The only options are to re-activate the deleted account in Bill.com or to contact Customer Support to request that the error is cleared.