It could be a classic Linux permissions error.

Make sure that folder is owned by the user that is running qbit. It really, really depends on how you did your setup or how you installed qbitTorrent, but a lot of times it creates its own user and you can have permission issues if that user cannot access that specific folder. For example, if the folder you were trying to download into was made by Root, a normal user with regular permissions wouldn't be able to access that folder.