An active workflow that includes a status without an exit transition cannot have one added. This is extremely onerous if the workflow is widely adopted, applied to issues types in multiple projects. You have to create a new workflow, and update each project's issue types that use it, rather than making a simple update to the workflow once.

The error reported is that you cannot perform this operation on a draft" but that is misleading. You ALWAYS work on a draft, even if the workflow was just published it becomes a draft by virtue of clicking the edit button.