To answer the original question: I think root user is the first/admin user that you signed up with Wasabi. If you create users after that, they are called sub-users. Depending on your setup, you have to create a key for the root user or sub user. I guess you can just create the key for root user if you are not using sub-users. See https://docs.wasabi.com/docs/creating-a-user-account-and-access-key