I get this too:
Element “link” is missing required attribute “property”.
page --> <link rel='stylesheet' id='su-other-shortcodes-css' href='https://www.traindocsrural.org/wp-content/plugins/shortcodes-ultimate/assets/css/other-shortcodes.css?ver=4.9.6' type='text/css' media='all'>
the problem is that the <link> tag is supposed to be in the <head> section of the page, not the <body>
