See “domain already taken” when you insert your domain under Settings?
This means that another site in an account you opened in IM Creator is linked to the domain you are trying to connect. It can also mean that the domain is already connected to your site, but hasn’t been published yet. Log into the editor of the site and click on PUBLISH.
You can disconnect a website from a domain by clicking on Manage under the thumbnail of the site which is connected to the domain and choose “disconnect domain”
See more hereץ