Not able to place an order after installing this extension

[Jackwilson2018]

Hi,

I just install this extension and configured the sandbox credential. When I place an order I am getting this error: "Please check your payment information and place the order again." on the checkout page. When I enable the debug the mode and getting this error message:

Class Pmclain\Authnet\MerchantAuthentication does not exist {"exception":"[object] (ReflectionException(code: 0): Class Pmclain\Authnet\MerchantAuthentication does not exist at /var/www/html/vendor/magento/framework/Code/Reader/ClassReader.php:37, ReflectionException(code: 0): Class Pmclain\Authnet\MerchantAuthentication does not exist at /var/www/html/vendor/magento/framework/Code/Reader/ClassReader.php:29)"} []

I have installed this extension with the Magento 2.2 version.