Unable to delete SharePoint Online sub-site. Sites that have subsites or certain apps can't be deleted.

Issue Details:

You can not delete a SharePoint site which has an active sub site under it. While trying to delete you see error occurred message.

SP hosted app (deployed by user)  is also technically created as a sub-site under the site where it is activated, the sub-site can not be deleted without uninstalling/deleting the app.

Solution:

In order to remove the sub-site, we have to remove all apps on this sub-site (Remember: Not from Site Collection). (It may ask you to go back to classic experience in SharePoint Online site).

Resolution Steps:

1. Click on gear icon, go to site contents 

2. Go to app 

3. App Setting

4. Remove/delete

 

Wait for few min until the message is shown. Now try deleting the site again and it should not show any issue now. 

:)