If you receive the error message 'this domain is already in use,' when trying to sign up for a Google service, it might be because:
- You recently removed this domain from another managed Google account. It can take 24 hours (or 7 days if you purchased your account from a reseller) before you can use the domain with a new account.
- You or someone in your organization already created a managed Google account with your domain. Try resetting the administrator password and we’ll send an email to the secondary email you provided when you signed up, telling you how to access the account.
- You’re using the domain with another managed Google account that you own. If so, remove the domain from the other account.
If none of these applies, the previous owner of your domain might have signed up for a Google service. Fill out this form and the Google Support team will get back to you within 48 hours.
Source: Google Workspace Admin Help