You are getting this error because you tried to subscribe with the same email address too many times and MailChimp has flagged it as spam/robot. This often happens during testing.
Use a new/different email address for testing.
Also, you can take a look at the official explanation from MailChimp:
If that helps, you can consider upgrading to PopupAlly Pro, which include a lot more templates and features. PopupAlly Pro also includes email support and we’ll be happy to help you out with any further questions you may have. We have a 30-day money back guarantee on the plugin, so feel free to take it for a risk-free try. If you don’t like it for any reason, just let us know and we will refund your purchase.