customfield cant set with Jira update Issue API ?

[itn.saurabh.chavhan]

 my task is to copy all the content of a issue and update another issue with all this content with all fields i successfully able to update other fields but there is a filed devolopement where merge request, pull request links are available this field is type string and when i try for this one  its saying 

{ "errorMessages": [], "errors": { "customfield_10000": "Field 'customfield_10000' cannot be set. It is not on the appropriate screen, or unknown." }}

[Muhammad Ramzan(Atlassian Certified Master)]

There could be two reasons

1) First and most appropriate reason is, please check the issue screen it looks this field is not available there, Simple way is , create the same issue from UI and see if the field exist there if exist, then check the ID is correct or not.

2) you don't have permission to create the issue or this field is restricted and you don't have permission