Hi Philipp,
The “add tax rate to checkout” option is what you’re looking for - the issue here is probably that you haven’t set up any tax rates on your stripe dashboard. In order for this option to work, it needs to know which tax you are looking to add.
You should be able to visit your stripe dashboard’s tax settings (https://dashboard.stripe.com/settings/tax) and set up a VAT rate for your app - note that while stripe currently supports dynamic and fixed tax rates, our plugin only supports fixed tax rates at this time.
Let me know if you have any trouble getting that set up!