Missing: best or worst rating (structured data)

Hi,

I’ve been trying to get my search results show up as rich snippets, but have not been successful so far. When I check the list of Structure Data in Google Webmaster tools, my recipes have a “Missing: best or worst rating” error.

Please help me resolve this, Is it a problem with Recipe Box, or something else? Let me know if you need any more information. My site is http://www.yummygraphy.com

https://ww.wp.xz.cn/plugins/recipe-card/