Hi Plummer, Katiya,
Thanks for reaching out to Microsoft Q&A.
Make sure you are a member of the Project Administrators group or have been granted explicit permissions to move work items, which can impact your ability to copy or clone work items. There might be issues due to the browser being used. Try executing the action in a different web browser or clearing the cache of the current browser.
When using the "Include child work items" option, make sure you are not trying to copy the user story to a different project or work item type, as this feature is disabled in those instances. This could cause the copy action to fail.
Clone the user story first without child tasks and then try to add child tasks manually or in smaller groups.
Hope it helps!
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If you have any other questions, let me know in the "comments" and I would be happy to help you.