That's caused by net terms orders.
The app is trying to redirect to a non existing page in your store, which results in a 404 error.
Head over to your offer > edit > scroll down > notice the net terms checkbox:
Notice the "Redirect to the following page after the order is created"
As such, the page will redirect to .myshopify.com/pages/thank-you
If you don't have a thank you page - follow this guide to add it:
Or, use a different address.
When redirecting to a valid page, you won't see any 404 page not found errors.