After updating your WHMCS application to the latest version, your transactions are no longer processed to completed status. This is due to the update process of WHMCS removing the callback file of PayFast.
This file is payfast.php.
When you download the plugin from our site, https://www.payfast.co.za/integration/shopping-carts/whmcs (The direct download link is here) this will download the entire folder.
Locate the payfast.php file within this folder and place it in the following locations:
modules/gateways/callback/
modules/gateways/
This is so that the file location looks like:
modules/gateways/callback/payfast.php
modules/gateways/payfast.php
Once this is done, please set the permissions to 644. This allows for read/write access of this file.
Once you have completed this, process a transaction and the callback function will now work.