Deleting a site collection is a fairly straightforward process for classic SharePoint; however, modern SharePoint sites that are connected to Office 365 groups require a few extra steps to be permanently deleted:
- Navigate to the SharePoint Admin Center (https://admin.microsoft.com), expand Admin centers, and select SharePoint.
- In the left navigation pane of the SharePoint admin center, select Active sites.
- Select a site to delete by clicking on it. If it is a site that has not been connected to an Office 365 group, you'll get a simple dialog to delete the site, as shown in the following screenshot:

If you selected ...