PayPal Checkout SSL Requirement
The PayPal Checkout payment gateway module requires an HTTPS-secured connection to create the link between WHMCS and PayPal correctly.
If the WHMCS installation's domain does not have a valid SSL certificate, the connection will not work.
To meet PayPal Checkout's requirements:
1. Make certain that you have correctly installed an SSL certificate.
If you need to purchase an SSL certificate, go to whmcs.com.
2. Update the WHMCS System URL setting to begin with
https:// in the General tab at Configuration > System Settings > General Settings.
For more information on updating the system URL, see Updating the System URL.