I have upgraded your user SiriK to trust level 4 (highest), so now you have all permissions except admin. Can you give it a try again, it should hopefully work now. I was not sure what issue you’re running into specifically and what “changing users’ activities” means, so I’m sorry I cannot help more directly so far.
If you’re trying to post content on behalf of other Discourse users, then the “Impersonate User” function in Discourse might be a better alternative. How are you doing it right now? “Impersonate User” is unfortunately an admin-only function and it’s never good security-wise to have too many admins (we already have). But if you need it, I can make you admin for a time.