Mcrypt has been removed from PHP versions 7.2 and newer. However you can manually install it yourself if you have a dedicated or VPS server.
For Ubuntu/Debian:
https://www.techrepublic.com/article/how-to-install-mcrypt-for-php-7-2/
For CentOS 7:
https://whattheserver.com/how-to-install-mcrypt-for-php-7-2-and-php-7-3-on-centos-7cpanel/