| rest /services/admin/passwords doesn't work on my Production system, but appears to give the same output as
| rest /services/storage/passwords on my dev system. Since storage/passwords is documented, I suggest you use that.
Your question is tagged with REST Endpoint Examples, but your comment says you are using the Python SDK. Which is correct?
storage/passwords is used to save user (those who sign in to Splunk) credentials.
I don't see admin/passwords anywhere. Can you post a link to it?