There is a known issue with ConnectWise where the PO Tax is orphaned when received. This has been logged with ConnectWise and is expected to be resolved in future release 2016.5. This is specifically due to the fact that the Purchase Order tax is referencing the Packing Slip (for Purchase Order Freight)
You will need to delete the batches for the PO's that are erroring and complete the following
1. Open the PO
2. Open the Purchase order lines, by clearing the "Close" checkbox
3. Repeat #2 for all lines
4.When all lines are open, uncheck the PO Tax Box on the front of the Purchase Order:
5. Close the PO, all lines will now re-close.
6. Ensure that any payments are removed the Bill in accounting package.
7. Re-Sync the PO to accounting package.