SD card won't work

So I recently got pokemon sun for my 2ds, and it said I need an SD card so I went to a store and got an SD card, but when I put it in it said that it had write protection, and guess what? I had the switch turned up! And I try almost everything, I turn it down and up a billion times, I take out the game and turn it in every different direction, and it works ONCE but the game was used so it had a save file, and I look up how to delete saves, and I try it again, but it didn’t work that time, so can someone help me please?