QuickBooks Desktop sync error: Only one AR or AP account allowed per transaction

Why the sync error occurs

This error occurs when the Unallocated Expenses Account selected in the sync preferences is something other than an Expense Account.

How to fix the sync error

Update the Bill.com sync preferences for Unallocated Expenses Account to an Expense type account:

In Bill.com:

1. Select Settings
2. Select Preferences under Sync
3. Select Edit
4. Change the Unallocated Expenses Account preference to an Expense account
5. Select Save