Question : The PayPal shows transaction is completed, but on the site, transaction status showing as 'Pending' at site backend

=> At the beginning of 2016, PayPal has introduced a number of security-related product updates. These updates are mainly used for improving security standards

Following are the updates:

1. TLS 1.2 upgrade (support for TLS 1.0 will be retired)
2. IP Address Update for PayPal SFTP
3. IPN Verification Postback to HTTPS (HTTP will no longer be supported)
4. Merchant API Credential Upgrade (to SHA-256 2048-bit credentials)
5. SSL Certificate Upgrade
6. PayPal SDK Updates

For more information refer the below link

Note: TLS 1.2 is only supported in OpenSSL 1.0.1c and later AND cURL 7.34.0 and later running on PHP 5.5.19+ or PHP 5.6.3+. If these conditions are met we can use PayPal's minimum requirement of TLS 1.2 which is mandatory since June 2016.

Question : When the User tried to do a transaction via PayPal Payment Gateway, the user is getting below message on PayPal Page as

 "Return to Merchant

We cannot process this transaction because there is a problem with the PayPal email address supplied by the seller. Please contact the seller to resolve the problem. If this payment is for an eBay listing, you can contact the seller via the "Ask Seller a Question" link on the listing page. When you have the correct email address, payment can be made at www.paypal.com."

=> If you are using the PayPal plugin as a payment gateway, then makes sure that you have configured the PayPal plugin correctly.

For PayPal, Plugin configuration refers the following link. https://techjoomla.com/documentation-for-common-payment-gatewaycpg-api-for-joomla/setting-up-common-payment-api-plugins-in-supported-extensions.html