Write protection enabled With no SD card?

I have a pretty old 3DS and just turned it on. When I did it stayed something like “write protection enabled software not able to be displayed”. I looked at the slot and saw no SD card. I am confused and also when I insert a card the 3DS acts as if there is no card. I know it works because it works on my DS. I am confused. I have also tried using rubbing alcohol but to no avail.