Checkout Payment Processing

I am trying to follow the information here for processing a credit card payment.

I am able to create a checkout and update a checkout. I am also succesful in vaulting the credit card. However, every time I get to Completing the Purchase, I get a 404 status/error from the payments endpoint.

I’ve verified that I am sending the session_id and unique_token and they look correct. I’m not sure what else would be causing the error.

https://help.shopify.com/en/api/guides/sales-channel-sdk/getting-started#request-payment-processing