The reason this invoice has failed to send because it has probably been entered manually in Xero and already has a credit note or payment assigned to it. 

First, go into Xero and find the invoice. Ensure that it exists correctly,

If the invoice exists correctly in Xero:

1. Go to Commusoft
   
   
2. Click Settings > Company Settings > Accounting interface 
   
   
3. You will see the accounting interface dashboard. Click View in the failed section
   
   
4. Click on the Invoices tab
   
   
5. Select the failed record, click Mark as do not send in the top right of this page
   
   

If the invoice doesn't exist correctly in Xero:

1. Go to Commusoft
   
   
2. Click Settings > Company Settings > Accounting interface 
   
   
3. You will see the accounting interface dashboard. Click View in the failed section
   
   
4. Click on the Invoices tab
   
   
5. Select the failed record, click Re-send records in the top right of this page
   
   
   

You will need to go and resend any payments and credit notes associated with this invoice, you can do this using the tabs across the top of the page and repeating step 5