Thanks for reaching out to us!
I've checked your account and found out that the site you're trying to activate the license is an legacy WordPress-plugin version (which is no longer supported).
Are you able to update the WordPress plugin to the latest version and migrate Buttonizer to your standalone account? This should be easy since you can follow the steps in the plugin when you're on the latest version, it's an automated process.
You should then be able to add the license to your site via our dashboard. If you'd like, we can also attach the license to your site as soon as you're migrated.
Hope to hear from you soon 🙂