Taking a look at their docs (https://apidocs.chargebee.com/docs/api?prod_cat_ver=2), it looks like they expose a REST API which you should be able to use in Retool..
You would use https://{site}.chargebee.com/api/v2/subscriptionsfor the Base URL.
You will also need to choose "Basic Auth" for your auth type, then put your API key in the username field.
Your configured resource should end up looking something like this:
Give this a try, and let me know if you run into any difficulty!