Hello,
From my knowledge this error usually means that the folder, otherwise known as a directory, has to be empty which means either you have installed fortnite before and forgot like I’ve done lol… or you have uninstalled it from the launcher as this person did in the fortnite forums (https://www.epicgames.com/fortnite/forums/battle-royale/royale-with-cheese/192390-fortnite-folder ). I hope this method works for you, open up the folders where fortnite is instaled by pressing the windows key on your keyboard aswell as R at the same time and it will open a so called “Run Dialog” in the text box enter the following, “C:\Program Files\Epic Games\Fortnite” without the quotes, if it opens up a window and you see folders named Engine and FortniteGame, then go into the FortniteGame folder and then go into the folder named Binaries then go into Win64 folder, scroll down until you see FortniteLauncher.exe , it should have the fortnite “F” logo on the left. Double click it,
If you do not see any files that have the fortnite “F” logo on the left then go back by pressing the blue arrow that points left on the top left of the window until you see a folder named “Fortnite”, hover over the folder with your white arrow and right click it with the button to the right of your mouse and click Delete.
If this helps you = Awesome
If it doesn’t, Message me back and I’ll be happy to help and possibly play fortnite with you if you have a PS4 
Sincerely,