Thank you for trying our plugin. This free plugin allows you to add CAPTCHA to native WordPress forms, such as the native login form, registration page form, comments form etc.
Your registration form is created with a third party plugin, so it is not a native form. In that case, if you want to add CAPTCHA to any non-native WordPress form you need the Premium edition of the plugin.
I hope the above helps. Should you have any further questions please do not hesitate to ask.
Have a great day.