Today I ran into an issue in SharePoint Online in which I could not create a new site collection using one of the classic templates. The SharePoint Administration Center allowed me to perform the steps to create the new site collection (many times, actually), but it never would actually provision it. After a bit of troubleshooting, I realized that I had initially created the new site collection using the modern experience, and I immediately deleted it. I then discovered that by default, SharePoint Online now temporarily holds deleted sites under an area named 'Deleted Sites,' and the only way to permanently delete the site collection is to run the following command in the SharePoint Online Management Shell.
Remove-SPODeletedSite -Identity https://company.sharepoint.com/sites/sitecollection
After running this command, the site collection is permanently deleted, and the URL suffix (alias) is now available for you to use again for your new classic site collection.
Have a great day.
