Description:
On the xero_error property of an order or invoice the following error is displayed and the invoice is not synced to Xero.
A validation exception occurred, The UnitAmount field is mandatory and cannot be derived from the Quantity and LineAmount fields., The LineAmount field is mandatory and cannot be derived from the Quantity and UnitAmount fields.
Reason:
The Xero sync will return this error if the order does not have a delivery date. A delivery date must be set before the order will sync.
Resolution:
Set a delivery date for the order. If the order has been paid this will need to be done via the order list view, otherwise you can edit the order directly on the order.
You may then need to manually re-sync the order to push it into Xero.