There are two ways to collect tax on invoices in Turnstile: automatic tax collection through Stripe, or manually adding a tax line to an individual invoice before it is issued.
Automatic tax collection
Automatic tax collection requires all of the following to be in place:
Stripe must be set as the invoice provider for the subscription (not only as a payment processor). You can confirm this in your invoice settings.
Automatic tax collection must be enabled on the linked Stripe account.
A default product tax code must be set on the Stripe account.
Manual tax collection
You can manually add a tax line to any individual draft invoice before it is issued. Open the invoice and use the edit invoice workflow to add a tax line item.
A Stripe integration is not required to add a tax line manually.