You might need to update your PHP version from 5.x.x to 7.x.x to make this work. Not sure if the plugin works with a PHP version that old.
FYI: before you change your PHP version, make sure to talk to your host before doing so since that can cause your site to crash depending on the code used in your theme, plugins and what not.
Make sure your WP version matches the plugin requirements. You can use
wp_config.php to turn on debug mode to find out if the plugin is causing issues.
You can read about that here:
Next is making sure you have done the conversion part before doing the export.
Make sure your PHP is set to allow a
file_size of minimum XX whereof XX matches the size of your entire post content. Start with 8mb and see where that takes you. Also, make sure that the time out is set to more than 30. You can ask your host for advice on this as well.
Please also make sure to read the official doc on this: