The button appears only when all of these are true:

• Your Stripe account is connected and Stripe has finished checking it. Account, Integrations should show Stripe as Active. If there is more to do, open Invoicing, Products and settings and press Finish setting up Stripe.
• The invoice has been sent (not a draft, and not void).
• There is a balance left to pay.

Card payments are not available in the demonstration account.