Watermark not being added

WordPress 4.5
Wocommerce Version 2.5.5

I have activated the plugin and added the watermark image and saved the settings. But it does not create the watermark images after clicking Apply Watermark.

Is there any minimum requirement of the server, for this plugin to work.
Like php version, GD library, Ghost Script etc.

https://wordpress.org/plugins/yith-woocommerce-watermark/