Missing from the Stripe screencast is the addition of the following script tag to the <head> section of the application layout file (app/views/layouts/applications.html.erb):
<script src="https://js.stripe.com/v3/"></script>
This script tag must be added to the <head> of section of the application layout for the Stripe checkout to function. Without it you will see a "500 Error" in your browser's developer console when you click on the "Buy Now" button.
See my version of the application layout with this script included here.
In order for the Stripe API to function you will need to set a company name in your Stripe account. Without this the API will only respond with a 500 error code. You do not need to supply Stripe with any banking or credit card details, just the name of a fictional company so that you can access the "sandbox" version of the API. When using this sandbox API you can use the following test creditcard number to make purchases: 4242424242424242