Huh! The code I posted works for me on Terraform Cloud. This seems like the same error from your other thread, which sure looks like a permissions problem.
My advice would be to try creating a new user token, and double-check that this user is an organization owner. If you still have see the same error, you might find it easier to debug using the Terraform Cloud API directly, and eventually contact Terraform Cloud support.
Hope you get it working—if you do, please post back here!