In Jira, some custom fields are multi-select menus, whereas in Rally, they are single select. The problem is the values don't get picked up. The error indicates that the values are not valid for the field
Release : SAAS
When the connector grabs the field value from Jira, it is generating a Rally API call that assumes it will be going into a multi-select field in Rally as well. Since the API syntax for setting a single value field is different from setting a value for a multi-select field, the Rally API fails the request.