error when paypalpayment is completed..

You have that problem:

When your server receives a notification it needs first to confirm whether this notification comes from your payment gateway or not because it is always possible that a valid IP address has been injected and that someone else is tried to validate an order. So there is a mechanism in place to contact a paypal server HikaShop knows as being an official paypal server to confirm that it comes from paypal. If this connection fails, that means that either your hosting company is blocking outgoing connections from your server or you don't have the openssl option activated in your PHP configuration or that the paypal servers are down, the later being a lot less likely. So you should check that you website is able to make outgoing connections and that the SSL is activated. Also, we advise you to contact your hosting provider to ask them about their policies on outgoing connections.