What do I do if Fortnite won't let me change my skin?
If you're experiencing an issue of not being able to save your outfits or equipped cosmetics after selecting them from your locker, this can be caused by having the “Shuffle Presets” option enabled in your locker. If it says "Disable Shuffle", click on it so it reads “Enable Shuffle” instead.
Can you get banned for making custom skins in fortnite?
As of Chapter 2, Season 3, there hasn't been any update that allows you to make your own skins in the game. If you somehow hacked the game files and made your own custom skin, then you would probably get banned in less than a week.