Google says this about Google Site names:
When selecting a URL for your site, keep the following in mind:
But that's not the whole story. Not only can your Site name not conflict with an existing Site name, it can't conflict with a Site name that has ever been used by another account even if that Site has been permanently deleted by the account owner.
Let's say I have a Google account firstname.lastname@example.org and another account email@example.com. I create a site called perjury98site using account firstname.lastname@example.org. I then realise the error I have made and delete and then permanently delete the site. I then try to create a new site called perjury98site using the account email@example.com.
In fact, no other account can ever use the Site name perjury98site. Only the account firstname.lastname@example.org