Hi,

Steps:

1. Fire REST Request: https://server.de/rest/api/2/issue/ABC-1234

   JSON: {"fields":{"customfield_123456":"something"}}
   
   

2. Error returned "400: "Field 'customfield_123456' cannot be set. It is not on the appropriate screen, or unknown."

Initially thought of a different reason for this error, apparently user does not have an 'Edit Issue' permission so with the same request, just granting permission works.

Jira Version: 7.7.1

Suggested error message: '401: "The calling user does not have permission to edit the issue.'

Issue similar to: https://jira.atlassian.com/browse/JRASERVER-44003
Derived from Support Ticket: https://ecosystem.atlassian.net/browse/DEVHELP-976