How to fix missing function in wp-cron?

You can also disable the auto wp-cron from loading and setup a manual cron job on your server.

Add the following line to your Wp-config.php
define('DISABLE_WP_CRON', true);

and then on your server/control panel create a cron job to run the following:

wget http://www.yourdomain.com/wp-cron.php > /dev/null 2>&1

I run mine every 5 minutes.