Wrongful error: (my site) "is already assigned to an account and in use on another account."

I looked up the domain and the answer is simple: the domain is already assigned to your hosting account. But it’s assigned as an Addon Domain, not a Parked Domain. If you want to have it as a parked domain, you’ll need to delete it from the Addon Domains list first. Also, I strongly recommend to use the client area to manage your account instead. The control panel has a lot of strange and confusing things like this. The client area just show the list of all domains on your account (instead of hiding them in three different menus). And if a domain cannot be assigned because it’s in use, it will actually tell you where the domain name is in use.