If you are unable to void an invoice, it may be because there are payments applied to the invoice. You must void any payments before you can void the invoice.
1. When you click on the 'Void' button, check to see if there is a message at the bottom of the window that says "Unable to void, there are payments associated to this invoice".
2. If this is the case, you should see a number in brackets beside the 'Payments' tab.
3. Click on the 'Payments' tab and you will see the payment(s) associated with this invoice. From here you can click on the number in the 'Id' column to open the payment form.
4. Void the payment(s).
5. Once all the payments have been voided, you should be able to successfully void the invoice.