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Before adding taxes and discount, you need to activate their settings.
(1) Go to Home > Click on Settings.
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(2) Then activate the Tax and Discount.
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(3) Then Open an invoice > Then click on a line item.
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(4) Then set the Discount and Tax values > Then click on the Save button
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(5) You can then configure if discount is before or after tax. From the invoice screen, click on the Discount section.
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(6) Check the Apply discount before tax if needed, then click on the Save Button.
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(7) This will automatically re-compute for tax and discount.