The most common cause of this error message is when an item exists in All Orders which was never created in Quickbooks and the item has not been assigned a proxy and is not in the sync queue waiting to be posted to Quickbooks. To resolve this issue follow these steps:
-Open All Orders as the admin.
-Go to the file menu, maintain, advanced tab, run db command, and paste in the following script:
insert into SyncQ (UniqueID,TxnTypeID)
select NCItemInventoryID,100 from ItemInventory where ListID is null and SyncQB=1 and Proxy is null
-This will add entries to the sync queue telling the system to create the missing items in Quickbooks. Perform a sync to create the items in Quickbooks.
-Try and post the transaction to Quickbooks again.