SharePoint Online helps you with automatically redirects, if you change the url for a SharePoint Site Collection. If you delete a site within the SharePoint Online Admin center, it might be that those redirects won’t be deleted. In this case you’ll try to use the site adress again and will be faced with a message like this:
In case you want to reuse the url you should start Powershell.
Connect-SPOService -Url https://yourdomain-admin.sharepoint.com Remove-SPOSite -Identity https://yourdomain.sharepoint.com/sites/siteadress
This two lines will free up the old redirect and will not blocking the site url anymore. All details can be found here at Microsoft Documentation.