When you're working with the CCSFG, it by default only allows you to work with new contacts and will not allow you to use email addresses that are currently, or have been previously in use on your account. If you were to test with an email address that has not been used in your account before, you should see the success page. The best solution for that is to use our new embeddable sign-up forms which you can read more about here: https://knowledgebase.constantcontact.com/articles/KnowledgeBase/18265-Create-a-Sign-Up-Form-with-the-new-sign-up-form-experience
I do want to specifically note that the new embeddable forms specifically do not have working support for success/failure redirects, but it is on our roadmap for the near future to implement a success redirect.
Please let me know if you have any questions!
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