Always an error message

Hi together,

I´m trying to put with the parallax scroller plugin an parallax effect to my website. And it does not work. I always get the message “No Feature Image Defined! Please specify your background image in the featured image meta box on the admin page. For more information please see the help menu from the parallax scroll admin pages.”

I worked with this tutorial: https://siteorigin.com/thread/how-to-add-parallax-scrolling-to-a-page-builder-row/ but it does not work. I work with the Zerif Pro theme. Does anyone know if there some problems with this theme when I´m using the parallax scroll plugin?

Thank you in advance!

https://ww.wp.xz.cn/plugins/adamrob-parallax-scroll/