After adding a payment input to your typebot you need to configure the input by clicking on the wheel next to it.
It is recommended to use the "test" key pair to test your typebot in preview mode. You can find your Stripe API test keys here.
It is also recommended to add at least an email (it can be a variable collected from a previous question). A receipt by default will be send to this particular email.
When you are ready to go live, don't forget to change your replace your test keys with the public ones.