Hi,
Have you made sure the relationship you are trying to delete is 'broken-off' (assuming you don't need it)?
Just run this command to sanity check the relationship's state:
::> snapmirror list-destinations
And depending upon their state, use the commands appropriately.
::> snapmirror break -destination-path <vserver>:<path>
::> snapmirror delete -destination-path <vserver>:<path>
::> snapmirror release -destination-path <vserver>:<path>
Also, take a look at this thread: Just for reference purpose.
https://community.netapp.com/t5/Data-ONTAP-Discussions/Can-t-remove-root-volume-for-DR-SVM-vserver/td-p/121211
Thanks!