What it means
The API answered with HTTP 403 and the error code private_item.
The item exists but is private, and the person this token acts as cannot open it. Only an admin of a workspace with admin privacy sees this answer; ask the owner or the person who made it to share it.
How to handle it
Every error is an RFC 9457 problem document (application/problem+json). Match on code, never on the message text. See errors for the full list.