Hi,
I have never worked with this software (SCOM), but I found some reference related to your query, may useful for you.
I found this link, which states: Webservices basically requires 'lcoal administrator account' rights.
It demonstrate all the steps, in case you have missed this one, plz take a look:
https://www.stefanroth.net/2012/09/09/scom-2012-netapp-management-pack-version-3-1/
It's an older version mentioned in the link, but I guess the logic is same.
Thanks!