Cause: The autoresponder connection or list ID is missing or incorrect.
Fix:
Go to the Autoresponders tab and verify your connection is active (look for the green "Connected" status).
Double-check that your List ID is correct. A single wrong character will cause this error.
If using AWeber Legacy, make sure you entered the Unique List ID (e.g., awlist0000000), not your list display name.
If using an API-based provider, click Test Connection to verify the API key is still valid.
Check if the funnel has a per-funnel autoresponder override that might be misconfigured.
Check the Debug Log in Settings for specific error messages from the autoresponder API.