PSID is unsupported GET request Graph API?

I'm trying to get labels associated with participants of conversations on my fb page.

My GET request: <pageID>/conversations?fields=participants,id&access_token=<PageAccessToken>

The POST:

    {
      "participants": {
        "data": [
          {
            "name": "Jim Bean",
            "email": "numbers@facebook.com",
            "id": "2821512231253160"
          },
          {
            <my page info>
        ]
      },
      "id": "<convo link>"
    }, 

When I click on the participant's ID, I get the following error.

    "message": "Unsupported get request. Object with ID '2821512231253160' does not exist, cannot be loaded due to missing permissions, or does not support this operation. Please read the Graph...",
    "type": "GraphMethodException",
    "code": 100,
    "error_subcode": 33,
    "fbtrace_id": "AwlTn9Vx3mfPtRGnquaHm8k"

Thus, I cannot request labels associated with PSIDs without being able to get info on my user's PSIDs.

What am I missing? How can I retrieve PSIDs from conversations?