Error installing magento "PHP Extension mcrypt must be loaded ..."

The mcrypt module has been deprecated since PHP 5.5 (at least several functions of it) and has been removed in PHP7.2, as per the link :

  

Warning This feature was DEPRECATED in PHP 7.1.0, and REMOVED in PHP 7.2.0.

     

Alternatives to this feature include:

     

• Sodium (available as of PHP 7.2.0)
  
• OpenSSL
  

This message is popping up for you because the version you are using Magento is either old or you are using PHP7 on your local machine.

There is no way to enable this module in PHP7, as it simply does not exist for PHP7 anymore and in PHP5 if it is Windows only by downloading .dll from some external source, however, I recommend that you upgrade Magento!

If it is Ubuntu and PHP 5 you can try to install via repository (maybe it is available) through the command:

 sudo apt-get install php5-mcrypt

Then:

sudo ln -s /etc/php5/conf.d/mcrypt.ini /etc/php5/mods-available/mcrypt.ini

Then enable the extension using php5enmod and restart Apache:

sudo php5enmod mcrypt
sudo service apache2 reload

If something fails, especially in apt-get , it's probably because Ubuntu has already removed this module from the repositories.