Fatal Error on Activation

Getting this while activating.

Any thoughts?

Notice: Use of undefined constant MCRYPT_RIJNDAEL_256 – assumed ‘MCRYPT_RIJNDAEL_256’ in /usr/share/nginx/html/wordpress/wp-content/plugins/smarts3/includes/class-methods-smarts3.php on line 8

Thanks!

https://wordpress.org/plugins/smarts3/