Re: Unable to change SharePoint site address

Check Recycle Bin sometimes after deleting a site, it might still be in the tenant recycle bin. You can use PowerShell to permanently delete it from there.
Clear-PnPTenantRecycleBinItem -url <site_url>
Keep in mind that it can take about 10 minutes to change the site address, depending on the site’s size. During this time, the site will be read-only.
It’s best to perform this change during low site usage or out of hours