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.
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.
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 CNAME records - any subdomain mapping CNAME records for the Google Site (typically www)
Delete Forwarding - If you have any domain or subdomain forwarding, delete that
Forward your root domain 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
Add your Custom Domain to your Google Site. This could be in the Google Sites Settings or Google Workspace Admin Console. It might be safer to wait 10 minutes before trying this so that domain forwarding/record propagate.
Go back to your DNS and add the subdomain CNAME record to connect your Google Site (typically www to ghs.googlehosted.com.)
If you want, you can go to Forwarding settings and forward the root domain to your full subdomain address.
This was completed with a paid Google Workspace account and Google Domains (now Squarespace Domains).
We will start with the Google Domains page, let me know once you're there.
6:21:26 PM : Alright. Please click on Manage Custom records.
6:21:52 PM : After that, click the trash icon for the www CNAME record.
6:21:57 PM : Once done, please let me know.
6:22:14 PM : Done
6:22:21 PM : Checking..
6:22:44 PM : Perfect! Now, click on Website at the left hand side.
6:23:02 PM : ok
6:23:03 PM : Click on Forward Domain.
6:23:21 PM : Under Forward to, please enter: google.com
6:23:34 PM : Then click on Forward.
6:23:56 PM : ok
6:24:15 PM : Checking... One moment.
6:24:40 PM : Awesome! Now, click on DNS tab at the left side again.
6:24:48 PM : ok
6:25:00 PM : Click on Domain Forward and click Delete.
6:25:36 PM : done
6:25:47 PM : 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.
6:26:00 PM : Awesome now, kindly go to the Admin Console.
6:26:01 PM : excellent
6:26:07 PM : ok
6:26:23 PM : Once you're on the page, let me know if you are able to select the www already.
6:26:32 PM : ok
6:27:26 PM : still indicated custom url already taken
6:27:51 PM : Can you close the page and open a new one?
6:28:01 PM : sure
6:28:31 PM : Alright. Let me know how it goes.
6:29:59 PM : Looks good (Pending)
6:30:06 PM : Thank you
6:30:18 PM : Great! Can you send me a screenshot of the page so I can check as well?
6:30:31 PM : My pleasure!
6:30:37 PM : Which page?
6:30:50 PM : Admin Console where you added the Custom URL.
6:30:59 PM : ok
6:31:03 PM : Thanks.
6:31:33 PM : 3.JPG
6:31:37 PM : Thanks again
6:32:01 PM : Awesome! Oh, we're not done yet though, .
6:32:11 PM : ok
6:32:51 PM : We will need to re-add the CNAME record and forward your domain to have it be visible.
6:33:04 PM : Alright. Do you still have the Google Domains open?
6:33:13 PM : ok yes
6:33:27 PM : Great! Please click on the DNS tab again.
6:33:38 PM : Click on Manage Custom Records.
6:33:50 PM : ok
6:34:03 PM : After that, click on Create new record, then add the following:
Name: www
Type: CNAME
TTL: 3600
Data: ghs.googlehosted.com
6:34:16 PM : Once done, click on Save.
6:34:51 PM : ok
6:35:06 PM : Awesome, now click on Website tab.
6:35:14 PM : Click Add a forwarding address at the top right corner.
6:35:39 PM : ok
6:35:50 PM : Click Edit across Forward From. Then, click the trash icon across www.
Under Forward to, enter: www.[YOURDOMAINHERE].com
6:36:19 PM : Screenshot 2023-10-10 9.36.10 AM.png
6:36:26 PM : The set up should look like that.
6:36:34 PM : Then click on Forward after,
6:37:30 PM : all set
6:37:36 PM : Checking...
6:38:08 PM : Perfect! Now, you're all set and your domain is already properly connected to your new Google Sites website.
6:38:15 PM : Please note that for the DNS changes to take effect, it might take a propagation period of several minutes depending on your location or up to 24 - 48 hours across the entire internet.
6:38:33 PM : Would there be anything else that I can assist you with aside from this for now?
6:38:34 PM : Great. Thank you
6:38:40 PM : That should do it