If you are unable to apply the credit notes in your Invoice, you will need to check if you have created the Credit Notes in the Accounting Tab.

Credit Notes need to be created first before applying to Invoices
  1. Go to the Accounting Tab on the sidebar menu.
  2. Select the Credit Notes’ card >> Click on ‘Create’ button
  3. Fill in the respective field such as the contact name, currency, accounts, tax if applied, and the exact amount to be refunded back to your customers.
  4. Click on the 'Save’ button.
  5. Once you have created the credit note, you can apply them to the respective customers on the invoice page