Difficulty setting a custom feild

The API documentation is great - however I am having an issue implementing a like example and could use some guidance.  Item 4. at https://www.jetbrains.com/help/youtrack/devportal/api-howto-create-issue-with-fields.html#step-by-step details how to create an issue and set a custom field such as "assignee" .  I can get the example to work as documented when assigning issues to myself, however the server returns a 400 with the error - "b'{"error":"","error_description":"Value is not allowed","error_developer_message":"Value is not allowed","error_field":"value"}'" - when any other team member's login is used (I can get a 200 back using my login in).
The relevant portion of the payload contents is

{'name': 'Assignee', '$type': 'SingleUserIssueCustomField',
 'value': {'login': 'dewayne.dahse'}},

The custom field structure is

All help would be appreciated