Introduction
Any issue that occurs while attempting to add or update a Mailchimp subscriber will often generate the same generic message
Unable to add/update subscriber: Invalid Resource
The error message is also saved as a note on the entry.
How To Capture More Info
Recreate the error with Gravity Forms logging enabled (see this article). The error will be saved to the Mailchimp error log, as well as a hopefully more detailed error message from the Mailchimp service.
For some Mailchimp errors, additional information is sent if a specific field has thrown an error. These errors are also saved to the Mailchimp Add-On log.
Checking Error Codes
You can cross-check any error code captured against Mailchimp’s Error Glossary
If the logged information does not help figure out why the error occurred, a developer could add specific debug statements into the site code to try and track the error.
But if you still cannot see any easy resolution, submit a ticket to Gravity Forms Support for technical assistance.
Example
This screenshot shows an example snippet from the error log.
Note how the log identifies what exactly was sent, and what the returned error was. In this case, it is clarifying that when using the Mailchimp Address field, Mailchimp expects that you pass a complete address, including street address and zip code (If you only want to collect the city/state, create a text audience field and merge tag in Mailchimp to map to your Gravity Forms city/state field).