Just before launch - Checkout error

Working on a new client site upgrade using FluentCart. They're launching the next product in a few days. But when testing the checkout today, I get an error. (From Stripe?)

What is this?

The cart has one fixed-price product + an optional bump-offer for a monthly subscription.

Possible to get this analysed/fixed before Sunday? If not, I will have to revert everything and have them go back to the old solution.