Hi @David ,
Welcome to Q&A forum!
This can happen if the site was not deleted properly and some files or pages are still present. You can try to restore the site from the recycle bin or the second-stage recycle bin by following the steps provided in the "Restore a deleted home page" section of the "Error: 404 File/Page not found" in OneDrive or SharePoint article. If the site is not found in the recycle bin, you can check whether the site was deleted or redirected by following the steps provided in the "Check whether the site was deleted" and "Check whether the site was redirected" sections of the same article.
Have a nice day!
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