I recently got the above error when trying to connect a custom domain I had bought through Google Domains with a Google Site. It seems that the previous domain owner had also used Google Sites and possible Google Domains too. The fix was actually relatively simple.

1. Log in to Google Domains, select the domain question and click on Website from the left hand menu.
2. Click on Web Forwarding and put a random URL (e.g. google.com) in the prompt and select Forward.
3. Click on DNS in the left hand menu and then delete the forwarding you just set up in the Synthetic Records section.

Now if you go back to Google Sites and try to connect the domain it should work!

Hope this saves you time with Google Support which I had to go through to work out this hack.

‘This URL is already in use by another Google Service’ Google Sites error was originally published in Google Cloud Platform by Cloud Ace on Medium, where people are continuing the conversation by highlighting and responding to this story.