How to set subscription order status Fulfilled, for non physical products

Hello, I’m developing a custom subscriptions app for my company.
For the moment, they just sell non physical products as subscriptions. The problem is that for each subscription, the resulting order status is UNFULFILLED or SCHEDULED (in case of billing attempts). I’ve created a node express API endpoint, but after spend some days in it I still getting the following error: “Request failed with status code 406”, and I’m not sure how to overcome this issue.
Here is the code I’m using: assets/fulfillment_service_route.ts at main · raycastsant/assets · GitHub and here is the complete error details file: assets/error.txt at main · raycastsant/assets · GitHub.

I’d appreciate any help you can provide to me. Thank you!