This article covers 

Can Create checkout to update a subscription API be used to reactivate/ resume subscriptions?

How can I reactivate subscriptions via API?

Summary 

The Checkout to Update a Subscription API call, cannot be used to reactivate or resume subscriptions. This API call is to be used for subscription changes, such as Plan changes, price changes, adding/ removing addons, etc. 

To reactivate a subscription via API, please make use of this API call → https://apidocs.chargebee.com/docs/api/subscriptions?prod_cat_ver=2#reactivate_a_subscription  

Solution

a. This API endpoint- https://apidocs.chargebee.com/docs/api/hosted_pages?prod_cat_ver=2#create_checkout_to_update_a_subscription is to be used to make changes to subscriptions, such as a change in price, change in product, etc; and not for reactivating a canceled subscription- irrespective of the Hosted Page version. 

b. Reactivating subscriptions using the above endpoint is not supported with the above endpoint; even with In-App checkout (Hosted pages v3), or Full page checkout (the version you are using- Hosted Pages v4). 

c. If customers wish to reactivate their subscriptions, you can: 

1. Allow them to head to the portal to reactivate their subscription, as given here--> https://support.chargebee.com/a/solutions/articles/256404 and https://www.chargebee.com/docs/2.0/self-serve-portal.html

2. Or, you can reactivate the subscription at your end (on their behalf), by using this API call- https://apidocs.chargebee.com/docs/api/subscriptions?prod_cat_ver=2#reactivate_a_subscription

More on reactivating subscriptions here --> https://www.chargebee.com/docs/2.0/reactivation.html