Recently, while trying to convert a case to work order we got the below error.
The immediate thought was to check all the workflow associated to work order and case and see if they all are published, which in fact they were.
Then as suggested here
http://ms-crm-2011-beta.blogspot.ae/2016/11/workflow-must-be-in-published-state.html
we deactivated and activated one of our SLA written against the Work Order entity and it fixed the issue.
Hope it helps..
