Returned when the requested crew member or vessel isn’t on one of your attached vessels — or doesn’t exist. The two are deliberately indistinguishable so you cannot enumerate the CrewPass user base. 
{ "error": { "code": "not_found", "message": "no crew member with that id on your attached vessels" } }
Fix
  • Confirm the vessel is attached to you (vessels/lookup returns attached).
  • Confirm the crew member appears in your GET /fleet results.
  • A crew member who genuinely exists but sits outside your fleet will always return 404, never 403.