I recommend you email support@asana.com, mention this thread, and see if they think you should follow this:
and then perhaps when you go to convert workspace to org, it will then succeed.
That assumes there’s nothing now in the org of value.
Note that Support needs to remove the org anyway, so you could do the steps in the linked posts (if you agree they’re safe) and then make that request of Support–to delete the org–if you and they both think that’s the correct approach; I’m not sure).
Thanks,
Larry