This error comes up after you have previously voided a supplier invoice and entered the invoice into Workshop Software again.
When you void a supplier invoice, the invoice sync over to Xero but in a voided state. So Xero already has a record of that supplier invoice number.
The only way to get around this issue at the moment is to copy the supplier invoice 81721400. See link for how-to – Copy Invoice function).
Once you have copied the supplier invoice put the same invoice number in the reference but with a random character at the end. So instead of 81721400 you would put in 81721400a. You still need to know what the actual reference number is but different enough so that Xero will accept it.
Once you have done this, process invoice 81721400a and void invoice 81721400
Press the Xero button in the top right hand corner.
This will perform a manual sync and push the copied invoice over to Xero.