In Odoo v15, you can add tax tags to the order line of a posted POS order by editing the order line and selecting the tax tags that you want to apply.

To edit the order line of a posted POS order, go to the "Point of Sale" menu and select the "Orders" option. Then, click on the order that you want to edit, and go to the "Order Lines" tab in the order form. In the "Order Lines" section, click on the order line that you want to edit, and go to the "Tags" tab in the order line form. In the "Tags" section, click on the "Add an Item" button, and select the tax tags that you want to apply to the order line from the dropdown menu.

Once you have added the tax tags to the order line, you will need to click on the "Save" button to apply the changes and update the order. You can then view the order line and see the tax tags that have been applied to it.

Keep in mind that you can only add tax tags to the order line of a posted POS order, and not to the order line of an unposted or draft order. This is because the tax tags are used to calculate the tax amount for the order, and the tax calculation is performed only when the order is posted.

I don't think you can do it since its posted !