Custom button's "Get shortcode" field is empty

I’m using a custom Likebtn theme for my Likes—I chose the lightbulb icon—and it’s showing up fine on the individual post page. I’m trying to get the shortcode for the custom button so I can add it to the template for the archive page. However, the “Get shortcode” field at the bottom of the button settings is empty. I tried resetting the button and that field is still empty. I’m just using the free plan for now; should this function be working? Thanks!

https://www.ads-software.com/plugins/likebtn-like-button/