Xero fault - means that invoice number already exists in Xero for the invoice you are trying to sync from Autotask.
This issue is usually caused by the option "Use Autotask Numbers" selected under invoice options.
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 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 sequential between the two platforms. For further information about setting this option up please click here
Need to resync an invoice please read our support article here