Xero error: This invoice/credit note number is already being used

First, go into Xero and find the invoice/credit note. Ensure that this exists correctly.

If the invoice/credit note exists correctly in Xero, go through the following steps:

1. Go to Commusoft.
   
2. Go to your company settings and open the 'Accounting interface' section. 
   
3. You will see the accounting interface dashboard. Click 'View' in the failed section.
   
4. Click on the tab corresponding to the type of record that has failed i.e Invoices/Credit notes.
   
5. Select the failed record, click 'Mark as do not send' in the top right of this page.

Please note that when you do this you will 'break the link'. This will mean you will need to manually enter any payments related to this invoice/credit note into Xero at a later date.