Magento – Authorize.net : An error occurred on the server

As defined in app/code/core/Mage/Authorizenet/Model/Directpost.php, this exception is thrown if any portion of the below evaluates as false:

if (!$this->getConfigData('trans_md5') || !$this->getConfigData('login') ||
    !$response->isValidHash($this->getConfigData('trans_md5'), $this->getConfigData('login'))
) {
    Mage::throwException(
        Mage::helper('authorizenet')->__('Response hash validation failed. Transaction declined.')
    );
}

So, it's one of three things:

• You have no trans_md5 defined in the config tree payment/authorizenet_directpost/trans_md5

• There is no login defined in payment/authorizenet_directpost/login

• The method isValidHash is returning false

The method isValidHash returns false because there is an issue with either the generated hash (e.g. your API Login/Key are wrong, generate a new one and re-enter) or the amount of the transaction somehow differs between Authorize and your system.

The latter is less likely, but because generateHash in Mage_Authorizenet_Model_Directpost_Response uses the transaction amount as part of the hash, it's a potential factor here.

Generally this message is handle from two core files:

Magento\Checkout\Model\PaymentInformationManagement and Magento\Checkout\Model\GuestPaymentInformationManagement

from function savePaymentInformationAndPlaceOrder if you want to change default message and display your exception message then just

throw new CouldNotSaveException(
            __('An error occurred on the server. Please try to place the order again.'),
            $e
        );

replace above line with below code

throw new CouldNotSaveException(
            new \Magento\Framework\Phrase($e->getMessage()),
            $e
        );