What this error message means.
This issue is usually caused by the option "Use Autotask Numbers" selected under invoice options.
This is a Xero fault. It means that invoice number already exists in Xero for the invoice you are trying to sync from Autotask.
The fault is caused because Autotask has generated a invoice number XXY and Xero already has a invoice number XXY that may have payments or credits attached to it.
A number of workarounds are available to resolve this. The easiest is to change the invoice number in Autotask and then click the resync button in Cloud Depot (please ensure the number you use hasn't already been used in Xero).
Please note: its important to configure your Xero and Autotask environment so that numbers are always guaranteed to be unique and don't cause duplicates. For further information about setting this option up please click here