In order to perform the migration, make sure you have Cron running as both Magento and the Cloudinary plugin need Cron jobs running.

For more information about setting up a Cron job in Magento:

http://devdocs.magento.com/guides/m1x/install/installing_install.html#install-cron

https://www.easycron.com/cron-job-tutorials/how-to-set-up-cron-job-for-magento

Note that you can use the plugin without migrating old images, however only new images added after you have enabled the plugin will be managed and optimized by Cloudinary.

In addition: