how to delete nonexistent messages in Outlook

[ghost]

I'm trying to follow Microsoft's instructions to delete a read receipt that's stuck in progress, here https://docs.microsoft.com/en-us/outlook/troubleshoot/synchronization/delete-nonexistent-messages . I get to step 6, "right click on root mailbox", but I don't have one, just root container. I've attached a screen print of what I've got in the hopes you can tell me where I went wrong. Their description is exactly what's happened to me, so I'd really like to fix the problem. thanks