Symptom:
When mapping fields in a follow-up action, the Output dropdown shows “Nothing available.”
Reason:
The Outputs were not re-added after the last test or the Custom Action was saved before the fields were mapped.
Solution:
Edit the Custom Action, click Run Test Request, open Show Full Response, and re-add the Outputs. Save the action before returning to the screen editor. Then reopen the mapping menu, and the Outputs should appear.