Without more information, it's difficult to help troubleshoot your problem. For new development with the subscription API, we recommend you start by following the tutorials. This is the section on selling plans and selling plan groups.
- Have you been able to create an order with a subscription contract for non subscription only products?
- How did to determine the order did not have any attached subscription contracts? Have you tried querying for the subscription contract off of the order?
- What is the definition of the selling plan you used?
- Have you registered for the SubscriptionContract webhook?