Hi
the GetDedupeSizeShared is not a real zapi call hece the tool uses the system-cli command with the options
set diag
sis stat -vserver <vservername> -volume <volumename> -field shared-data
you can try to run the same command with the target user.
then as far as i can see you did not define all the required privileges for the discovery role.
Please dobule check. For example you have to add the system node run as all access and other.
here the list on page 28
https://library.netapp.com/ecm/ecm_download_file/ECMLP2371573