W3C compliant: "Element link is missing required attribute property"

When I’m trying to validate my website: andreamate.com in W3C validator I’m getting this error:

Element link is missing required attribute property

Then I’ve checked my code and the error is found in this script:

<link rel='stylesheet' id='wp-customer-reviews-css' href='https://www.andreamate.com/wordpress/wp-content/plugins/wp-customer-reviews/wp-customer-reviews.css?ver=2.4.6' type='text/css' media='all' />

Anyone is having this problem too?

https://www.ads-software.com/plugins/wp-customer-reviews/