You can add a discount to an invoice when either creating the invoice or when editing the invoice.

New Invoice

You will go to the "Finance>Invoices" tab to add the discount when creating an invoice.

Click on the "+ New Invoice" button.

You will see the box to add a discount.

Existing Invoice

To add a discount after the invoice has already been created, you will go to the "Finance>Invoices" tab.

Find and click on the invoice you'd like to edit via the pencil icon.

You will then see the box to add the discount.