WHMCS is a leading web hosting automation system. Using WHMCS, we can manage billing as well as support system using it.
As of WHMCS Version 6.0, a new feature is added PayPal Subscription Automation. A new option has been added under Setup > General Settings > Invoices tab called Automatic Subscription Management. Once enabled, WHMCS will automatically cancel the PayPal Subscription ID associated with a service at service cancellation and upgrade time.
This option will work on if you have started PayPal Subscriptions after 2009. If you have old PayPal account (Subscription ID start with “S-“), this function will not work until your PayPal account is converted by PayPal. You can contact PayPal merchant team to convert your account.
If you are unable to cancel PayPal subscription using this new option, most likely you have old PayPal account (with “S-” ID).
I was dealing with this issue since long time but I was unable to find the root cause. Now it is clear to me that why it was failing to cancel PayPal subscription from WHMCS.