Fix Google Sites Custom URL Already Taken Issue
Are you trying to add a custom URL from your verified domains, but the web address is already in use or taken? Let me explain the solution so that you can add your web address by freeing up the subdomain location.
Why Your Custom Web Address is Already in Use
It may be that at one point you had mapped a Google Sites property to this custom URL and so it's already taken. This could be from a Gmail account or Workspace account with that primary domain. Actions you can take before a more drastic solution:
Check Classic Sites & New Google Sites properties
Old personal sites, perhaps using other Google/Gmail accounts
Browse automatically converted New Google Sites you may have forgotten about to remove the custom web address assigned to it.
You can't assign two sites the same URL, so this creates a conflict. If you can't find the site to delete it, or remap it, it is particularly problematic.
How to Unblock the URL
We had a conversation with Google Sites support to solve this problem and here are the steps to clear the old mapping:
Open up your DNS Records / Advanced DNS Editing Interface (from your Domain Registrar) or Forwarding Options
Delete subdomain mapping CNAME records for the Google Site (typically www)
Forward your site to google.com; Forwarding the domain to google.com is usually our way of refreshing your website records connected to the domain so after doing do, we can now connect the new Sites URL through Admin Console.
Wait 2 minutes
Delete all site forwarding, including the google.com forwarding we just did
Go to the Admin Console of Google Workspace
Try to add your Custom URL again. It might be safer to wait 10 minutes before trying this so that domain forwarding/record propagate.
Don't forget to readd a subdomain CNAME record (typically www to ghs.googlehosted.com.)
After this you can delete that forwarding and set it up new to make sure your naked domain is forwarding to your subdomain site (typically www).