Note: You can only delete folders which do not contain any subaccounts.
To delete an empty folder, click the three dots next to the folder and then "Delete Folder".
You will get a confirmation message (or an error message) at the bottom of the screen.
If the folder can't be deleted because it contains subaccounts, you must first move any open or closed subaccounts out of that folder. Here are instructions on how to move subaccounts between folders: How do I move a subaccount between folders?