Payment fields grayed out in form

I would like to add Stripe to a form to collect payment for products, but all of the payment options in Form Elements are grayed out and not active. Why is this? 

 

It means that one of the payment tools is already added to the form. So you will be able to add the Stripe payment field once you remove the current payment field.

Removed products will be saved in the form, so they will appear in the Stripe field once added. However, the ordered products will be removed from the old submission data. It means that if you would like to keep the old orders' data, you can use a new form.

1) Clone the form.

How to Clone an Existing Form from Your Account

2) Remove the payment field in the cloned form, and then add the Stripe field.