If you are having difficulty creating a new folder, it could be that there is a folder with the same name already in existence.
Try auditing your folders by going to the File menu > Subscribe...
Select your account in the pulldown menu, then review your folders. If you already have a folder created with the same name, consider subscribing to it so that it is visible in Postbox.
Once you've made your adjustments, remember to restart Postbox for the changes to take effect.