These settings are only available when you use the plugin without WooCommerce.
Go to the Tax rates tab in the Invoices → Settings menu.
You can add new tax rates, sort and delete rates from the list. The first item in the table will be the default invoice rate.
The tax rate name will be displayed on the invoice and the rate will be used for calculation.
If you turn on WooCommerce, the plugin will use tax rates from WooCommerce.
Check the documentation about Settings for VAT-exempt Entities and Settings for tax payers.
You may also read more in the article on How to set up tax rates in WooCommerce for EU invoices.