Topic
When a running workflow instance exists for a deleted list item (i.e. the item was deleted before the workflow completed), it is not possible to terminate the workflow through the site.
Instructions
To terminate 'orphaned' running workflow instances, run the following command from the SharePoint Management Shell:
NWAdmin.exe -o SyncTerminatedWorkflows -url <SiteURL> -terminateDeletedItem
Additional Information
Attempting to terminate a workflow through the graphical user interface (GUI) may result in the following error:
soap:Server was unable to process your request.--->Item does not exist. It may have been deleted by another user.
NWAdmin Guide
https://help.nintex.com/en-US/nintex2016/sp2016/NWAdmin/aa-topnwadmin.htm