Please refer our documentation, where access to view ticket fields is available to authorized folks who can view ticket fields or create a new ticket.
Note: The agent whose credentials (API key or username/password) are being used to make this API call should be authorised to either view the ticket fields or create a new ticket.
Also, this API is deprecated as of June 20th, 2020. Suggest you to use the new API - v2/ticket_form_fields.