When a field is marked as required, Jira will check to make sure that the field has a value when you are making updates to the issue, such as an edit, or during a workflow transition. This validation is applied even if the field is not present on the screen, so make sure you do not make a field that is not required on screen; otherwise, users will not be able to complete the action.

Certain fields, such as Assignee and Due Date, require the user to have certain permissions to make updates. If the user does not have the necessary permissions, the validation will fail, and prevent the user from completing the action.