Element link is missing required attribute property

The error comes up because the css is on the body and not the header.
I solve this by using a plugin called WP total cache, it has a “minify” section, in there you can insert the url of the css and it will remove it from the body and put it on the header it will also minify it, you can even add as many css files as you want, instead of having many css all over your site you will have 1 unique minified css on the header.
Try it!

see this link
https://coolestguidesontheplanet.com/add-javascript-css-files-head-wordpress/