Hello @yutotwinky,
Since our plugin grabs the information it displays from WooCommerce, your older orders need to be modified since they do not contain any taxes.
Since it’s not actually the invoices you need to add tax to – but the orders – if you add the taxes to the order, they will automatically appear in the invoice as well. The problem is that:
1) paid orders in WooCommerce are not editable
2) automatic tax recalculation of an order in WooCommerce assumes prices are ex-tax.
You can overcome the first point by adding this small code snippet to your site:
add_filter( 'wc_order_is_editable', '__return_true' );
If you haven’t worked with code snippets (actions/filters) or functions.php before, read this guide: How to use filters
After adding this you can edit line items of existing orders and you will also see the blue ‘recalculate’ button, but you should first enter the ex-tax price and then press that button.

This can be done more easily & automatically with the help from the combination of the Premium Templates extension, thanks to the Customizer (Custom function block) & a code snippet:
