Order by meta key numeric does not work

I use Filter Everything PRO. I created an ordinary product attribute called “rank” which has numerical values. This is a standard product attribute, not a Custom Field. I want to sort products based on this attribute. I created a Sorting Widget with the following configuration: 1) Order by > Meta key numeric; 2) I extracted the Meta Key from the “edit attribute” page URL: site.com/wp-admin/edit-tags.php?taxonomy=pa_rank&post_type=product. I used “pa_rank” as Meta Key. The Sorting Widget does not work, it returns an empty page with the message “no products were found matching your selection”. What have I done wrong? Have I used a wrong meta key? Thanks.