In the create-checkout-session.php file you will find the following code commented out

// 'automatic_tax' => ['enabled' => true],

Uncomment this line of code and the sales tax will be automatically calculated during the checkout.

Make sure you previously went through the set up of Stripe Tax: Set up Stripe Tax and you have your products and prices updated with tax behavior and optionally tax codes: Docs - Update your Products and Prices