To enable Compare products feature, access Customizer > WooCommerce > Compare and make sure to enable it there.
The 2nd important thing you will need to do is to set a page which will be used to display the products being compared. Make sure to create a new basic page (with or without any content) using the “Create page” button.
Finally, choose if you want to display the Compare buttons in the catalog or product pages and customize the buttons looks.
A short demo can be seen at https://d.pr/v/JOJ5Ox .