Hi Aamir,
Removing the workflows from a category does not have any effect on the active tickets that were created when the category still had workflows linked to it. To resolve your issue, you have to go to the ticket and make sure all the workflows are attached to the case are completed.
Alternatively, you can change the status of the Change Order record to 'Cancelled' status, this will the set incomplete workflow statuses to canceled as ll. After this canceling the CO, you then be able to close the CO.
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Kind Regards,
Brian