Below you can find a list of common errors, their causes, and steps to resolve:
Contact #XXX - Contact cannot be emptyPlease check if the Contact # referenced has a Name or Company Name set. Sometimes, there may be a space character in one of the name fields causing this error.
Contact #XXX - Another contact has this contact number. Please enter a different contact number.
Invoice #XXX - Account code must be specifiedPlease check that you have a 'Default Account Code' set on the module's settings page in WHMCS.
Invoice not of valid status for modificationYou should manually update the 'Next Invoice to Sync' number to the first invoice that has not yet been sent to Xero.
Invoice #XXX - Document could not be foundThis means that you are attempting to sync a payment for an invoice that is not yet in Xero. You will need to sync/create this invoice or increase the 'Next Transaction to Sync' number to skip over it.
Invoice #XXX - Payment amount exceeds the amount due on this invoice