The 404 means that it can't find the payment. It seems it either doesn't exist or you're using the wrong URL. I can't help you much further with that. Make sure you double-check the docs if you missed anything!
Looking at the docs I see that all payment IDs start with PAY-{some random string}. I don't see that in your example!
Documentation: https://developer.paypal.com/docs/integration/direct/payments/paypal-payments/#c2cb-target-1