As mentioned, Here
When you delete an Office 365 Group, the associated SharePoint site is sent to a recycle bin. You can see the list of Deleted Sites (aka sites in recycle bin) by going to the newer/preview SharePoint Admin Center. In order to be able to recreate a new SharePoint site with the same name, you used before you need to delete the old site from the recycle bin. To do so you will need to use PowerShell. Look for the "Remove-SPODeletedSite" command on Microsoft Docs for more information on how to remove deleted sites from the recycle bin. As long as your old site is in the recycle bin, you won't be able to create a new site with the same name. I also recommend running the Get-SPODeletedSite command in PowerShell to see all of the sites in your recycle bin, it may have more listed there than are showing up in the Admin Dashboard.
References:
- Using PowerShell Remove-SPODeletedSite
- Using Pnp Clear-PnPTenantRecycleBinItem
- From SharePoint Admin Center permanently-delete-a-site
Here Reusing a Deleted SharePoint Site Nameis a Similar post that might help you.