Hello there,
Please review your form settings first and make sure they are correct and save the settings even if there are no changes to be made, then check if the issue persists. If it does, then double check you API key and/or email address in the email marketing provider account to make sure it is correct.
Then enter a new API key or Auth code (deauthorize Optin Cat first from account settings if its Aweber) from the email provider and enter into the API key field (paste as plain text) and make sure you are getting a green check mark in the API key field instead of a red X mark. If you get a green check mark, then select a list from the “Select List” dropdown if your email provider supports it. Then check again if the issue persists.
If the issue still persists, then there might be a conflict in between Optin Cat and your currently active them or other active plugins. I will send you the conflict isolation steps in the next email once you confirm that you cannot resolve the issue following the above steps.
Regards,