Stripe Error: "Error: Failed to process the order, please try again later. Message: Invalid positive integer"

You are probably trying to purchase the subscription product, that has a trial period and no sign-up fee.

This product will be purchased properly as an upsell (from upsell page), but not as the initial product (from checkout page). Because technically Stripe API needs a non-zero charge at first to create a customer's object and save his card details into it. 

We would suggest you to set up a sign-up fee of at least 1 unit of your currency to fix this issue, if you want to provide a trial period to your customers.