Deleting an Existing Folder

Only Administrators can delete an existing folder.
You CANNOT delete the Default or the All Campaigns folder because they are system folders.
You can only delete EMPTY folders.

To delete an existing folder:

In the folder tree to the left of the screen, select the folder you want to delete. In the workspace to the right, a Trash icon and a Pen icon appear next to the folder’s name.
Select the Trash icon. The Delete folder dialog box appears.
Click Delete folder.

Note that you can only delete EMPTY folders. In other words, you CANNOT delete folders that contain:

Campaigns (both active and archived).
IMPORTANT:
Currently, archived campaigns are not shown in the campaign list unless the Show Archived checkbox is selected:

If the folder has archived campaigns and the checkbox Show Archived has NOT been selected, the folder could seem empty. However, it is not empty and thus the system will not allow you to delete it. The Trash icon will be disabled in this case.
Subfolders. You will need to delete each subfolder before you can delete the folder.