Validation Error - Due Date for Bills/Sales Invoices must be between 1 and 31

It is possible in Xero to set a default due date criteria which is invalid.  This appears as a blank entry for the number of days to calculate the due date.

When MechanicDesk attempts to sync the invoice/bill, the incorrect setting will trigger a sync fail for any related documents.

To correct this issue, edit the contact's payment terms in Xero to a valid setting (the default setting is '0 days after issue date', i.e. COD).  Once changed, start a quick sync from MechanicDesk to retry the affected documents.

There is a similar setting on an organisation level, this could prevent all documents from syncing with the same error.

Note that this setting does not affect due dates sent from MechanicDesk, as our sync sets the due date based on the settings configured in MechanicDesk.

Xero article on the settings mentioned above